Crazy Transit Pitches

[TheRatmeister]

but the 10mph and 15mph estimates seem high to me.

To be honest it's a very rough guesstimate, but if it's high I don't think it would be high by much.

If the C branch can average 8.5MPH with 11 traffic lights and up to 11 stops between Cleveland Circle and St Marys St, (3.8 lights or stops per mile), 10MPH with 2.5 lights/stops per mile (same number, 5 each for both again) between South St and Redstone plaza seems fairly reasonable. (If I've done my napkin math right, assuming a linear relationship between average distance between lights/stops and speed, a better estimate would be 13MPH. I'll keep using 10 to be conservative.)

Likewise, the southern segment is around 1.8 lights per mile, so 15MPH felt about right. (Assuming the same linear relationship it could be around 18MPH).

I'm actually slightly more confident about the southern segment because of just how few crossings there actually are. Assuming short tunnels are built at Foss Park and the Medford Supercollider, plus a couple minor left-turns removed, there are only 6 crossings between any tunnel or viaduct at Roosevelt Circle and the GLX. (Salem St, Fellsway Split, Central Ave, Riverside Ave, Grand Union Blvd, and Pearl St. (The removed ones are at Presidents Landing and Middlesex Ave).

And of course, this is an area where grade-separations at crossings would be very easy, with the exception of probably Pearl St and Grand Union Blvd. Effectively extending the dedicated ROW all the way to the Wellington Bridge (and therefore raising the average speed to around 23MPH, the same as the D branch) would be fairly easy.

 

[TheRatmeister]

I certainly could be wrong about this, but I went way down a rabbit hole here a couple of years ago, and I think this location also doesn't work. The problem is that the circumferential service (Aqua Line, or Bronze Line below) still needs to cross from the south side of the mainline tracks to the north. (Probably needing to traverse the BET.)

View attachment 58417

No matter where you do it, the Fitchburg Line needs to be crossed; the advantage of the East Somerville alignment is that it lets you do the crossing at one of the narrowest points:
View attachment 58418

(I'm a little less confident about my solution for Medford <> GLMF yard access as I've sketched it above, but I think it could still work somehow.)

You could build a cutting like this for the circumferential service. The station would be similar to your proposed McGrath station, built in a shallow cutting and then presumably covered over to deal with all the train yard noise.

I'll call each proposal by the station names, so Inner Belt for this one and McGrath/East Somerville for yours. Here's a brief comparison

Inner Belt	McGrath/East Somerville
No tunneling under buildings, but tunnel is longer	Shorter tunnel, but it needs to go under buildings
Hits both GLX branches with one stop	Hits both GLX branches with two stops
Two new stations: One elevated (D/E) and one below grade (UR)	Three new stations, One elevated (UR East Somerville), one at grade (D McGrath), one below grade (UR McGrath)
Alignment would not allow interlining with D/E	Alignment would allow potential interlining with D/E

In terms of which one is better, I think it mostly comes down to how tunnel costs work out. My intuition that a simple capped cut under the tracks would be cheaper, even though it's a bit longer, but that isn't based on much. The fact that you also need another set of platforms at East Somerville to hit both GLX branches is also a cost-negative.

If you value interlining the McGrath/East Somerville approach is clearly better, but as long as GLX is using low-floor vehicles I think that's not a good idea.

 

[Riverside]

I'm a little reticent to wade into this -- this particular corner of the Green Line Reconfiguration really just isn't that "fun" for me at this point, and I think it, more than perhaps any other component, will hinge on the outcomes of professional studies; I think there's a definite limit to what can be done here from a non-professional perspective.

But I also want to respond in kind to your substantial engagement with the topic! So I'll see what I can do

You could build a cutting like this for the circumferential service. The station would be similar to your proposed McGrath station, built in a shallow cutting and then presumably covered over to deal with all the train yard noise. View attachment 58471

This is an interesting idea, and is a good example of something that ultimately will just have to be vetted for feasibility from a professional perspective.

To some of your specific points:

McGrath/East Somerville: Shorter tunnel, but it needs to go under buildings

I don't think it should? There should (barely) be enough room for a 82' curve to go around the building on the end of Fitchburg St:

Alternatively, it might be possible (and simpler) to route both tracks to the far side of the viaduct (probably expanding the ROW a little bit). This would then leave the other track in place for easier yard access:

Stations: I agree that a station at Brickbottom Junction is pretty enticing from a network simplicity perspective. I think it would lose some points for (or would require increased cost to accommodate) pedestrian access from Inner Belt Road. (Access requiring either a pedestrian overpass or a tunnel.)

I'm also a little less optimistic about fitting in platforms along the viaduct; a 230' inbound platform would stretch from the merging switch all the way to where the tracks run right up against the edge of the ROW:

If the viaduct itself ends up requiring modifications to accommodate the new platforms, then that will also drive up your costs. Again, not fatal, but raises questions for me.

Inner Belt vs East Somerville: one item to call out about the value of an East Somerville Urban Ring station is that it would eventually enable transfers from the Lowell Line via a new infill. Combined with an infill at Sullivan, that would provide all northside Regional Rail lines with a single transfer to reach Kendall Square. (Fitchburg -> Porter -> Kendall, Lowell -> East Somerville -> Kendall, Eastern Route -> Sullivan -> Kendall.) I suspect that would actually result in substantial relief to the Green + Orange Lines at North Station. An Inner Belt station offers no transfer opportunity for Lowell.

McGrath vs East Somerville: if value-engineered to the bone, McGrath station could probably go (or be saved for a subsequent phase):

• Riders from downtown can use Medford service and transfer at East Somerville (although most of them will probably have transfered at Park already and used Red to cross to Cambridge)
• Riders from an extended Union Branch can transfer at Porter instead
• Riders from Union Square proper should have a strong local bus service to reach Kendall (less than 1.5 miles away)

That leaves riders who want to go between Sullivan(/Everett/Chelsea) and Union(/Porter/Watertown). I'm sure that isn't a trivial demographic, but I also imagine it's not the largest demographic.

(Also, once the Union -> GLMF yard lead is built, you could also institute a Sullivan/beyond <> Union/beyond OSR, if you really wanted to.)

Interlining and incrementalism: the East Somerville option provides additional flexibility in the long term, enabling interlining without requiring it. I also think that the East Somerville option is worth consideration simply on the basis of being a way to enable a Lechmere <> Sullivan service, should that ever be desirable; the likeliest use case for that service (IMO) would be as part of an Everett radial branch of the Green Line. And while yes, I am not in favor of interlining the Green radial service and the Urban Ring service, rolling stock is inherantly impermanent, and it's pretty easy to imagine a future where both the radial and circumferential services use high-floor LRT. For a large infrastructure investment like this, those kinds of long-term concerns seem worth consideration.

And yeah, this comes back to my original point: no matter how you do it, this is going to be an expensive project. I think it's worth spending that money on a robust design, and ultimately I do think the McGrath/East Somerville option has more benefits on paper (which, yes, also will drive up the cost). But also, going back to my other original point, this is definitely an area that would need proper study to say anything concrete (no pun intended), so I'm definitely trying to keep an open mind.

 

[TheRatmeister]

I don't think it should? There should (barely) be enough room for a 82' curve to go around the building on the end of Fitchburg St:

Reading things is definitely helpful, oops. You might have more utilities mess compared to the rail yard but even then the length probably makes up for it.

If the viaduct itself ends up requiring modifications to accommodate the new platforms, then that will also drive up your costs. Again, not fatal, but raises questions for me.

Oh for sure, if that needs to happen to any significant degree that would probably kill the idea.

Inner Belt vs East Somerville: one item to call out about the value of an East Somerville Urban Ring station is that it would eventually enable transfers from the Lowell Line via a new infill. Combined with an infill at Sullivan, that would provide all northside Regional Rail lines with a single transfer to reach Kendall Square. (Fitchburg -> Porter -> Kendall, Lowell -> East Somerville -> Kendall, Eastern Route -> Sullivan -> Kendall.) I suspect that would actually result in substantial relief to the Green + Orange Lines at North Station. An Inner Belt station offers no transfer opportunity for Lowell.

This could be fixed by building one CR infill at Community College instead, transferring to a Chelsea UR branch. (Or the Everett Branch too if you don't mind a bit of a detour and digging out a bit of Rutherford Ave.) Even with the Tobin ramps it seems like you should have no trouble fitting 3 island platforms there.

And yeah, this comes back to my original point: no matter how you do it, this is going to be an expensive project. I think it's worth spending that money on a robust design, and ultimately I do think the McGrath/East Somerville option has more benefits on paper (which, yes, also will drive up the cost). But also, going back to my other original point, this is definitely an area that would need proper study to say anything concrete (no pun intended), so I'm definitely trying to keep an open mind.

Agreed.

 

[Longfellow]

Not a criticism of this plan moreso a lament that a lot of these proposed stations are in an area that has yet to see development. Though that's part of why an urban ring is important, to get people out of inner belt and brickbottom and into kendall/elsewhere. There's a bit of a weird zone (not sure how to show this on a map) in Camberville where you are >10 min east from the red line and >10 min from lechmere or union square. Despite most of the housing being in those areas. And then the buses in those areas come every 40 minutes. Its a little frustrating.

 

[BosMaineiac]

For the sake of humoring the T’s planning decisions, what if in order to speed up the circumferential service progress the “Gold Line” is in fact an FRA-spec BEMU/EMU between West Station and Chelsea? In a pros/cons argument, wouldn’t this be more enticing in the short term compared to a light rail line? The Inner Belt area in particular would be a lot easier to traverse if this option was followed through with. I’m imaging this as an S-bahn style service that would probably be able to achieve heavy rail capacity much easier than light rail ever could, especially given the constraints with the legacy Green Line system.

I get it this may be an unpopular decision on here, but wouldn’t having this type of service sooner than later be the way to go? Feel free to roast me

 

[JeffDowntown]

For the sake of humoring the T’s planning decisions, what if in order to speed up the circumferential service progress the “Gold Line” is in fact an FRA-spec BEMU/EMU between West Station and Chelsea? In a pros/cons argument, wouldn’t this be more enticing in the short term compared to a light rail line? The Inner Belt area in particular would be a lot easier to traverse if this option was followed through with. I’m imaging this as an S-bahn style service that would probably be able to achieve heavy rail capacity much easier than light rail ever could, especially given the constraints with the legacy Green Line system.

I get it this may be an unpopular decision on here, but wouldn’t having this type of service sooner than later be the way to go? Feel free to roast me

Not roasting, but the general opposition to this approach is all the grade crossings in Cambridge. Cambridge is going to go nuts over all the streets being blocked at S-Bahn frequency train service. Light rail might be able to go over or under some of those crossings.

 

[BosMaineiac]

Not roasting, but the general opposition to this approach is all the grade crossings in Cambridge. Cambridge is going to go nuts over all the streets being blocked at S-Bahn frequency train service. Light rail might be able to go over or under some of those crossings.

It’s my understanding that Main, Broadway, and Binney streets will have to be at-grade regardless of mode. It looks like there’s enough room to bridge/pass under Mass Ave for either mode as well. I think the main argument is that light rail will be able to interface with traffic better than main line rail can. Could better timed traffic signals be able to facilitate traffic flow with the addition of grade crossings? I feel like that problem would need to be sorted out with either light rail or main line rail.

 

[JeffDowntown]

It’s my understanding that Main, Broadway, and Binney streets will have to be at-grade regardless of mode. It looks like there’s enough room to bridge/pass under Mass Ave for either mode as well. I think the main argument is that light rail will be able to interface with traffic better than main line rail can. Could better timed traffic signals be able to facilitate traffic flow with the addition of grade crossings? I feel like that problem would need to be sorted out with either light rail or main line rail.

Yes, and Cambridge and Gore Streets. You might be able to close Binney (It has been closed or restricted a lot in recent years due to construction, and people seem to adapt). Main, Broadway, Cambridge and Gore are going to be issues. Light Rail does give you a grade advantage potentially making some crazy alternatives possible (and a Mass. Ave. duck under easier).

 

[Riverside]

For the sake of humoring the T’s planning decisions, what if in order to speed up the circumferential service progress the “Gold Line” is in fact an FRA-spec BEMU/EMU between West Station and Chelsea? In a pros/cons argument, wouldn’t this be more enticing in the short term compared to a light rail line? The Inner Belt area in particular would be a lot easier to traverse if this option was followed through with. I’m imaging this as an S-bahn style service that would probably be able to achieve heavy rail capacity much easier than light rail ever could, especially given the constraints with the legacy Green Line system.

I get it this may be an unpopular decision on here, but wouldn’t having this type of service sooner than later be the way to go? Feel free to roast me

I wrote a bit about the Grand Junction Corridor a couple of months ago, and sketched out a somewhat similiar "minimum build", in response to the latest study. That study showed relatively low ridership for a West Station <> Eastern Route service. It also would have non-trivial disruption to ops on every northside line. (Lots of interesting detail in there, tbh, it's one of the better summaries/analyses I've seen of the drawbacks of mainline passenger rail on the GJ.)

@F-Line to Dudley outlined a sketch for LRT on the corridor a few years back and I think it still holds up.

The problem with any mainline rail is that you get very little wiggle room at grade crossings. The design has to assume the worst case scenario, that the train is unable to be stopped for any reason -- if there is a mainline train on the tracks in that signal block, you have to assume that it is in motion and will run over anything in its path. Like the honey badger, the mainline train doesn't care:

(This is because mainline trains require a much longer distance to brake, with the exact distance depending on the speed and weight/length of the consist.)

Light rail, by contrast, is explicitly designed to work in environments where it may need to stop quickly -- where it isn't possible/feasible to always yield the right-of-way to the train.

IMO, it actually comes down to timetable fragility. Yes, in theory you could configure the lights on Main St etc to accommodate a regularly timed mainline crossing. But what happens if the train is delayed? Light rail can be "cooperative" in a scenario like that, and basically be like, "Yes, normally transit-priority-signaling means that you cars & pedestrians would get a red light while I barrel through without stopping; however, since I am late, I will standby for 45 seconds until a less disruptive point in the light cycle." Mainline rail, by contrast, has no choice but to honey badger through, at which point the benefit of the precision timed crossing has gone out the window.

I'll get off my soapbox in a moment, but just to step back and kinda state something sorta obvious: there's a reason we stopped running mainline trains through grade crossings in downtowns, like, 150 years ago.

Most trains are like honey badgers: to be admired, respected, and reasonably feared, appreciated from a distance (or behind a barrier), given plenty of space to do their honey badger thing. We wouldn't foster a herd of free-range honey badgers in Kendall Square, and we shouldn't run frequent mainline trains through there either, for many of the same reasons.

 

[TheRatmeister]

It’s my understanding that Main, Broadway, and Binney streets will have to be at-grade regardless of mode. It looks like there’s enough room to bridge/pass under Mass Ave for either mode as well. I think the main argument is that light rail will be able to interface with traffic better than main line rail can. Could better timed traffic signals be able to facilitate traffic flow with the addition of grade crossings? I feel like that problem would need to be sorted out with either light rail or main line rail.

There is an alternative, and that alternative is elevated. Running over Albany St and then Main St lets you bypass the buildings preventing grade separation of the crossings.

 

[Riverside]

@F-Line to Dudley, I was doing some "ArchBoston spelunking" over the weekend and saw some very old comments of yours (12-15 years ago) where you referenced a study showing that a reactivated Millis Branch would actually have shockingly high ridership. The only one I could find is this one: https://archive.org/details/millis-commuter-rail-extension-feasibility-study-1998/page/n9/mode/2up

And from what I can tell, the ridership seems okay but not amazing; looks like each station would be similar to Lincoln or Shirley, which of course still are worthwhile commuter rail stations, but not what I would imagine as the most competitive. Do you remember if there was a different study you were referencing? Or do you have a different interpretation of the linked study?

(Why is this in Crazy Transit Pitches, one asks? "Single track extension of GLX to Needham with hourly/half-hourly extended runs to Dover/beyond." Which I had been noodling on, coincidentally, and looking increasingly skeptical at the vast density cavity; but then I came across your comment and wondered whether I should reconsider.)

 

[themissinglink]

@F-Line to Dudley, I was doing some "ArchBoston spelunking" over the weekend and saw some very old comments of yours (12-15 years ago) where you referenced a study showing that a reactivated Millis Branch would actually have shockingly high ridership. The only one I could find is this one: https://archive.org/details/millis-commuter-rail-extension-feasibility-study-1998/page/n9/mode/2up

And from what I can tell, the ridership seems okay but not amazing; looks like each station would be similar to Lincoln or Shirley, which of course still are worthwhile commuter rail stations, but not what I would imagine as the most competitive. Do you remember if there was a different study you were referencing? Or do you have a different interpretation of the linked study?

(Why is this in Crazy Transit Pitches, one asks? "Single track extension of GLX to Needham with hourly/half-hourly extended runs to Dover/beyond." Which I had been noodling on, coincidentally, and looking increasingly skeptical at the vast density cavity; but then I came across your comment and wondered whether I should reconsider.)

As someone who currently lives in Medway, getting into the city can be a huge pain in the ass, and a Commuter Rail extension to Millis would be amazing for mobility in this area. The gap between the Franklin Line and the Framingham Line is kinda large for an area so close to the city, and a lot of people choose to drive in on 109 instead of trying to take the T.

But it's admittedly hard to justify an extension to Millis based on density, especially considering the NIMBY opposition from Dover residents. IMO, any extension to Millis should be routed via the Franklin Line, which would avoid Dover and the Needham Line altogether, in addition to utilizing existing infrastructure like the Framingham Secondary. At Walpole, trains would turn north onto the Framingham Secondary, with a stop in Medfield Center (instead of an isolated park and ride at Medfield Junction). Trains would then head west into Millis, stopping in the middle of town at the Cliquot site. Putting a station at the Cliquot site is probably a better choice than reactivating the historic station, as Millis has been eyeing a mixed-use redevelopment at the Cliquot site for a couple of years (see attached image). A layover yard could potentially be located at this disused property, which is currently the end of the tracks to the west of Millis Center. It wouldn't be cheap to extend a branch of the Franklin Line to Millis, but it would be cheaper and easier than an extension of the Needham Line via Dover.

At a minimum, the Medway GATRA shuttle needs to be expanded with better frequencies. There should be a new stop in Millis somewhere (previously considered but never implemented), another new stop in the center of Medway at one of the shopping plazas, and a final new stop at Choate Park or Medway Community Church, either of which would be a better terminus than the existing stop at Medway Middle School.

Potential mixed-use redevelopment at the Cliquot site:

 

[TheRatmeister]

(Why is this in Crazy Transit Pitches, one asks? "Single track extension of GLX to Needham with hourly/half-hourly extended runs to Dover/beyond." Which I had been noodling on, coincidentally, and looking increasingly skeptical at the vast density cavity; but then I came across your comment and wondered whether I should reconsider.)

If you could get some Battery or hybrid LRVs, I think it could work. Electrification of the line would probably tip any cost/benefit back towards the "no" column.

 

[ritchiew]

(Why is this in Crazy Transit Pitches, one asks? "Single track extension of GLX to Needham with hourly/half-hourly extended runs to Dover/beyond." Which I had been noodling on, coincidentally, and looking increasingly skeptical at the vast density cavity; but then I came across your comment and wondered whether I should reconsider.)

Yeah, I can see some problems, but a neat idea. Any details you'd like to share?

My first though is just that green line trains are slow, so this trip would take a long time for that long distance. The current allowed top speed is what? 35 mph? With trains designed to max out at 50? Those trains aren't comfortable enough for the hour+ trip to downtown. Unless you saw some other practical use case?

 

[TheRatmeister]

My first though is just that green line trains are slow, so this trip would take a long time for that long distance. The current allowed top speed is what? 35 mph? With trains designed to max out at 50? Those trains aren't comfortable enough for the hour+ trip to downtown. Unless you saw some other practical use case?

Type 10s should be capable of 50MPH, which would be fine for the line. With an average line speed of around 30MPH it would take roughly an hour to go from Medway or Millis to downtown, reasonably competitive with driving.

 

[ritchiew]

Type 10s should be capable of 50MPH, which would be fine for the line. With an average line speed of around 30MPH it would take roughly an hour to go from Medway or Millis to downtown, reasonably competitive with driving.

That sounds overly optimistic. Today it's 40 minutes for the 8 miles from Park Street to Newton Highlands (averaging 12 mph). From there to Millis adds another 13 miles, far more than doubling the length of the line. It would be faster in that section, but averaging 30mph doesn't sound reasonable. The fastest historical average speed I can find on any rapid transit line is 21mph on the Red Line. I think Millis to Park Street would be at least 80 minutes, and it wouldn't be comfortable. Google says that same drive is 45-70 minutes, even in the morning, so the train wouldn't be very competitive.

 

[TheRatmeister]

but averaging 30mph doesn't sound reasonable.

Riverside-Newton Highlands averages 23MPH today, with a 40MPH max speed. JFK-Braintree averages 26MPH with the same max speed. With wider stop spacing (Probably one stop in Dover, one in Medfield, two in Millis, and two in Medway) and a 50MPH line speed, 30MPH average seems perfectly reasonable. That would mean Newton Highlands-West Medway in around 40 minutes.

Today it's 40 minutes for the 8 miles from Park Street to Newton Highlands

We're assuming GLRC here though, which would shave roughly 10 minutes off this trip time by way of avoiding the current central subway mess. (This whole pitch makes no sense if you don't assume that, since without it you'd need to add the GL Needham branch by cannibalizing the other branches which won't happen.) That would bring the total travel time from West Medway to 70 minutes.

so the train wouldn't be very competitive.

With no traffic it's a 55 minute drive, with rush-hour traffic it's anywhere from 70-100 minutes. For commuters it would definitely be a viable alternative.

and it wouldn't be comfortable.

Why not? Take a few Type-10s and fit them with more transverse seating for longer trips and you're all set.

 

[Riverside]

As someone who currently lives in Medway, getting into the city can be a huge pain in the ass, and a Commuter Rail extension to Millis would be amazing for mobility in this area. The gap between the Franklin Line and the Framingham Line is kinda large for an area so close to the city, and a lot of people choose to drive in on 109 instead of trying to take the T.

But it's admittedly hard to justify an extension to Millis based on density, especially considering the NIMBY opposition from Dover residents. IMO, any extension to Millis should be routed via the Franklin Line, which would avoid Dover and the Needham Line altogether, in addition to utilizing existing infrastructure like the Framingham Secondary. At Walpole, trains would turn north onto the Framingham Secondary, with a stop in Medfield Center (instead of an isolated park and ride at Medfield Junction). Trains would then head west into Millis, stopping in the middle of town at the Cliquot site. Putting a station at the Cliquot site is probably a better choice than reactivating the historic station, as Millis has been eyeing a mixed-use redevelopment at the Cliquot site for a couple of years (see attached image). A layover yard could potentially be located at this disused property, which is currently the end of the tracks to the west of Millis Center. It wouldn't be cheap to extend a branch of the Franklin Line to Millis, but it would be cheaper and easier than an extension of the Needham Line via Dover.

At a minimum, the Medway GATRA shuttle needs to be expanded with better frequencies. There should be a new stop in Millis somewhere (previously considered but never implemented), another new stop in the center of Medway at one of the shopping plazas, and a final new stop at Choate Park or Medway Community Church, either of which would be a better terminus than the existing stop at Medway Middle School.

Potential mixed-use redevelopment at the Cliquot site:
View attachment 58673

Yeah, I agree that Medfield + Millis + Medway is a better fit for a commuter rail branch off of Walpole. That would also provide access to jobs in Norwood and Dedham, and, if timed right, could provide access to Patriot Place via transfer. Despite linking Boston and Providence, the Providence Line doesn't actually follow the BosWash buildup between the two cities; the Franklin Line does, so it makes sense that an orientation towards it would be useful.

(Loathe as I am to admit it, I do think a PnR at/near Medfield Junction might be necessary; with such low density, there will still be a lot of people driving to the train, and building more parking in Medfield Center would be counterproductive. It looks like there's a... gym? Office building? Near the Route 27 crossing. Could make for some TOD + parking. [All the moreso if you can acquire the industrial lot on the other side of the street.])

Yeah, I can see some problems, but a neat idea. Any details you'd like to share?

My first though is just that green line trains are slow, so this trip would take a long time for that long distance. The current allowed top speed is what? 35 mph? With trains designed to max out at 50? Those trains aren't comfortable enough for the hour+ trip to downtown. Unless you saw some other practical use case?

The idea isn't super well developed, but yeah, the idea is to convert the extant ROW between Needham Junction and Dover's downtown to light rail. It's actually not that long: only 3 miles.

That sounds overly optimistic. Today it's 40 minutes for the 8 miles from Park Street to Newton Highlands (averaging 12 mph). From there to Millis adds another 13 miles, far more than doubling the length of the line. It would be faster in that section, but averaging 30mph doesn't sound reasonable. The fastest historical average speed I can find on any rapid transit line is 21mph on the Red Line. I think Millis to Park Street would be at least 80 minutes, and it wouldn't be comfortable. Google says that same drive is 45-70 minutes, even in the morning, so the train wouldn't be very competitive.

Riverside-Newton Highlands averages 23MPH today, with a 40MPH max speed. JFK-Braintree averages 26MPH with the same max speed. With wider stop spacing (Probably one stop in Dover, one in Medfield, two in Millis, and two in Medway) and a 50MPH line speed, 30MPH average seems perfectly reasonable. That would mean Newton Highlands-West Medway in around 40 minutes.

I think TheRatmeister is correct to point out that the overall average speed of the Green Line is misleading when applied to a route like this one. The Central Subway is slow. But when you look at speed on the Riverside Line itself, whose character is much closer to a hypothetical Dover Branch, the speeds are much faster. I just checked, and today the Red Line averages 37 mph between JFK/UMass and North Quincy, despite having an overall average speed of about 19 mph across the whole line; the outer Riverside Line is currently around 23mph but historically has been as high as 25 mph.

But even in the "worst case scenario" where the Dover Branch is slower than Riverside, traveling at an average of 20 mph, the three miles would take 9 minutes, and at 30 mph (comparable to the D Line between Newton Highlands and Chestnut Hill), it would be just 6 minutes. Even with a forced 5 minute transfer at Needham Junction, I think that would still put the overall journey time at just about 1 hour, which seems competitive to me. (And that's without taking into account GLR improvements like a Huntington Subway, as @TheRatmeister points out.) Through-running would (I think) get that journey down to 50 minutes.

(I know you two were discussing running time all the way out to Millis, which I do think would be unnecessary when compared to a Regional Rail branch as described above.)

As for the Dover Branch itself, the <10 min travel times means that single tracking should be sufficient to achieve Regional Rail-equivalent half-hour headways. Adding a small midpoint passing siding would enable 15-min headways, if desirable.

Electrification would be the big question. I would not want the T to invest in light rail BEMUs (or River Line-style diesel light rail) just for this little line, and I can't really think of any other corridors that would call for rolling stock like that. (Maybe something with the Marblehead Branch? But that's a big reach, and of course located entirely on the other side of the region.) So, yes, full electrification would be needed for this, and yes there are many projects whose priority I would put above this one. That being said, one of these days I'll actually sit down and looking at our per-driver spending on roads, and I'm guessing that 3 miles of single track catenary will actually look pretty reasonable by comparison.

As for stations: one option is to just have a station in downtown Dover, probably in the lot current occupied by the Town Garage.

Historically, there was a station at Charles River Village (I'm guessing around here). The 1998 study linked upthread assumed that there would be no station at this location, given its extremely low density, especially compared to Needham's four (!) other stations. I tend to agree; there will almost certainly never be major development here, and the walkshed will remain very light in population.

In favor of a station is the availability of a (small) parking lot (on a lot where it could potentially be expanded), which is otherwise scarce along the ROW. This location also happens to be almost exactly on the midpoint of the line's three miles, so building a simple station could be a parsimonious way to take advantage of potential holds at the passing siding. And the site is adjacent to a 30-acre park along the banks of the Charles (a bit larger than the Public Garden), which could make for a decent leisure destination.

 

[Riverside]

Which brings me to the part of this idea that delights me the most: imagining riding a trolley into the Needham Town Forest. The ROW runs along the Forest's southern border, with trails abutting at several points:

(I do think service oriented toward the Forest would need to be more frequent. However, at less than a mile, this service would have exceptionally short running time -- it's a similar distance to Newton Highlands <> Newton Centre, which the Green Line clears in less than 2 minutes today. So it would not be difficult.)

And while I expect this proposal would have the best cost-benefit calculation with a full extension to Dover, I think a very short extension just to the Forest could be worthwhile on its own.

(I say all of the above without knowing conditions "on the ground" with any particular detail. It looks enticing on a map, though.)