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.