How unrealistic would it be to double track every commuter rail line? And then triple or quadruple tack the 5 or 6 busiest lines to allow express trains? Would doing this make any tangible difference in terms of speed and reliability of the commuter rail?
It's not necessary, because capacity doesn't really match 1:1 with number of tracks.
Background. . .
In the old days of steam, 1840-1950, block signaling was fairly primitive and RR tracks were signaled mostly unidirectional. This was because:
- Steam locomotives were unidirectional and had to be turned around at wyes, loops, or a turntable. No such thing as passenger push-pull or freight cab-backward running, except with early electrics.
- The RR was mechanically dispatched from employees in lineside towers every umpteen miles pulling levers on a mechanical switchboard while reading a telegraph feed. The only way to keep it sane was to segregate track directions over-rigidly.
- Trains could run "wrong rail" if they absolutely had to for reaching a siding or junction, but it was kludgy at best and required extra cautions that gummed up traffic in the other direction.
- Track switching was primitive until switch motors were perfected, so throwing a crossover switch either required a human switch tender in the field (usually practical only at staffed stations or very busy spots), or complicated pneumatic machinery to auto-throw a switch.
Consequently, the only single-track lines in existence were short low-traffic branches and very ends of lines where "track occupancy" rules (i.e. one train holds total control of the entire territory until it either leaves the territory or safely ties down on a siding) were enough. Very busy lines like the innermost NEC, B&A, Old Colony, and Western Route were all tri- or quad-tracked because the scarcity of crossovers meant there had to be express + local tracks in each direction for express of skip-stop overtakes.
After WWII things were significantly modernized with bi-directional diesels, and what's called Centralized Traffic Control (CTC) signaling which consolidated all those manual field towers (last one on the T: Waltham Tower controlling Fitchburg Line, retired 2011) in favor of massively centralized and heavily automated master controls. The T has the whole northside and whole southside under centralized dispatch offices...and CSX, for example, dispatches close to 100K miles of its whole east-of-Mississippi route network out of some giant operations bunker down South. With that centralization the lines were updated with full bi-directional signaling and a crapton more automatic switches so no human had to physically touch traffic.
The upshot was that the extra tracks were no longer needed. Dispatch now had the whole 'big-picture' railroad at its fingertips instead of having to play telephone with a bunch of field towers, automated crossovers were way more numerous, and the signals (including widespread deployment of stop-enforcing cab signals) were much safer and more precise for staging close train meets. A well-designed 2-track line would have MORE raw capacity than the 4-track express+local line of old that could only do brute-force overtakes. And the single-track line with very well-placed passing sidings corresponding to scheduled meets could fare just as well as a double-track line. So the RR's, which were flailing around in bankruptcy by the 50's and 60's, ripped out most of their extra trackage...some haphazardly, some judiciously. Quad-track really only remained on the NEC where passenger traffic New Haven-south remained too insanely dense (even during the ridership famine era) to mess with. That's also when all of the non-passenger branchlines that were still human/tower-controlled had their signals retired and went dark (local examples: Cape Cod mainline, Framingham Secondary, Fitchburg Secondary, Peabody Branch, Fall River & New Bedford Branches + pre-restoration Middleboro & Plymouth).
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What does it mean today?
- A lot of the remaining single-track needs to be backfilled back to double due to changing traffic levels.
- They obviously re-doubled the Haverhill Line between Wilmington and Lawrence because the cuts to single were too severe 50 years ago.
- Franklin has a funded package starting construction soon for DT spanning the long stretch between Walpole and Norfolk stations so train meets are less brittle. It'll get a little bit more between Norwood Central and Windsor Gardens if Foxboro full-time service comes to pass.
- Worcester got the Beacon Park constriction filled, and is funded for Worcester Union Station to get a second platform track. That will make it the only line other than NEC & Lowell that are unbroken DT end-to-end.
- Fairmount's due to get its Readville platform located off the single-track connector so it can host intended service levels on a double-track full-high island. That would make Fairmount the 4th line to be unbroken end-to-end DT.
- RER will require a systemwide audit on where additional double-track is needed.
- Wildcat Branch will probably need to be doubled up if Haverhill trains are being moved out there.
- Reading Line Urban Rail requires Reading Jct. in Somerville to be doubled, the Wellington passing siding to be upgraded, and Reading Station-proper to be doubled. The unexpandable Medford-Malden single-track along the Orange Line is OK so long as long-haul service to Haverhill vacates.
- Newburyport Branch will need most (maybe not 100%) of its remaining single backfilled.
That said, some places can probably suffice as single-track for a very long time.
- Old Colony branches. Obviously until you spend megabucks to fix the Dorchester-Quincy pinch there's not enough traffic that can get through to merit unbroken DT. You'll just be adjusting the lengths of the passing sidings if scheduled meets change.
- Needham. Same reason; service levels can't physically increase because of SW Corridor congestion, so no point. The only contiguous DT it'll ever see is from Orange & Green.
- Gloucester-Rockport and Franklin-Forge Park. Arse ends of the line, so even at RER :30 levels trains clear the single track without problems. At most, Gloucester Station-proper and Franklin Station-proper could use a doubling-up instead of the double cutting out right before the stops...but it won't be needed on the running track between there and the terminal stops.
- Extremely short single-track exceptions. Examples:
- There's a practical reason why the Fairmount-Franklin connector is a single-track bridge that wasn't doubled-up. That short stretch spans "Readville Upper" and "Readville Lower" junctions, where the sheer complexity of the switches makes it operationally easier to stage as single rather than double. It's too short a stretch to affect ANY :30 or :15 headway on Fairmount, Foxboro, or Forge Park so is a non-issue.
- There may have to be a short stretch of single in the middle between Windsor Gardens and Walpole, because there's a narrow historic tunnel under a block of Walpole streets that would prevent side emergency exit from a stalled train if it went back to DT.
- Junctions: Wildcat Branch split from Lowell Line immediately crosses a grade crossing; no problem if it doesn't fan out to DT until after the crossing. Ditto the Middleboro/Plymouth split because both feed immediately into crossovers at Braintree Station and switches for 2 x 2 splits are maintenance-intensive.
For MORE than 2 tracks. . .
Basically nowhere except the NEC. And that's for one very specific reason: speed differential, not express vs. local. Even though you have a pretty dense layer cake of services envisioned for Worcester and even Lowell...with increased Amtrak presence bouncing around in there...all traffic is moving at more or less the same top speed. A well-designed crossover layout is what makes it happen.
So for example:
- Most Worcester service is going to be skipping through Allston-Newton like today and picking up only the occasional stop. It's the only way to keep sane travel times. On Amtrak, Framingham's the first stop for a Lake Shore Ltd. or an Inland Route train, so their needs for skipping over the Urban Rail Riverside locals are exactly the same as most Worcester schedules. One track layout between Yawkey and Riverside Jct. handles all meets.
- After the Riverside trains have dropped off, it's only Amtrak and some super-expresses that have to leapfrog anything in Wellesley and Natick. Fewer crossovers necessary for juggling that traffic, so there's more flex to add them if Amtrak's and T skip-stop's needs start to diverge.
- After Framingham, it's just Worcester and Amtrak. Other MetroWest locals have all dropped off. And Worcester is making the local stops everywhere except maybe on some future Heart-to-Hub schedule where they're making the same skips as Amtrak. It's pretty much crossovers for Amtrak.
^^Unless you raise the top speed >80 MPH, which will take some work, the meets happen pretty much the same spots whether they're Amtrak>T or T>T. It's mercifully straightforward. Things would be slightly different if Amtrak could rev it up to 90 MPH, because then the meets wouldn't be happening at the same crossovers and you'd need a more complex railroad. But in no practical sense is there anything here calling out for 3 tracks.
Now, you DO have places for 3-track passers: West-Boston Landing where it actually exists, the new Natick station which leaves a provision, and possibly a sorting stretch between Riverside Jct. across 128 if it's necessary for resetting the order after the Urban Rail trains turn out. Likewise, the Framingham and Westborough freight yard leads space out *possible* passing opportunities to the west if traffic ever gets hectic enough. But I would not consider any of those available lengths of triple necessities; some are just easy-grab luxuries because they're already present. Those wouldn't be needed for base-build RER. That's more for when you've got RER, NSRL, max-expansion Amtrak schedules, Framingham-Northborough branch service, and a couple decades of mind-blowing growth on the whole lot of them.
Q: WHY IS THE NEC DIFFERENT?
A: THE NEED FOR SPEED! 8)
The EMU'd Providence Line of our dreams is not going to look a ton different from today. You might kiss 90 MPH for an insignificant number of seconds, but generally speaking the stop spacing doesn't allow for much more than that (thus, not many EMU's on the market ever get rated for speeds >90 because the added expense won't be put to good enough use).
But NE Regionals sustain a maximum-rated 125 MPH from about a mile south of Canton Viaduct all the way to the RI state line. And Acelas sustain 150 MPH (to-be 165 MPH) from the curve by Sharon Station to East Junction in Attleboro. And at those speeds it gets incredibly difficult to time an overtake with any precision, because even a variable station dwell like if the Providence train's staff had to assist a special-needs passenger at Mansfield @ 1-2 min. penalty can be enough that the nonstop Acela has moved too far vs. the next crossover opportunity. Therefore, Amtrak dispatch has to level punitive and unilateral priority on its own trains to keep commuter chaos effects from screwing them up.
^THIS^ is where you need 3-4 tracks, because the speed differential so limits the effectiveness of crossovers for meets, and HSR ops limit how many crossovers you can feasibly spray around like a confetti cannon. Sustained triple-digit speed Amtraks that are at many times running >2x the speed of commuter locals need segregation. And the T needs to trim the fat out of the Providence schedule imposed by Amtrak dispatch priority. Until recently that called for a contiguous 3 tracks everywhere except certain pinch points like Canton Viaduct or the Blackstone River Bridge into RI (both non-issues), and 4-track bulb-outs at most stations. With the recent NEC FUTURE study claiming even higher HSR service levels post-2040, now it looks a lot more prudent to just quad everything up except by the Viaduct and South Attleboro. With slower-speed territory between Forest Hills and 128 just needing it because of sheer nuttiness of traffic levels in the RER era.
But unless we start to see a widening speed differential on other lines (not very likely) or start to see Metro North-level schedule congestion on other lines (also not very likely)...there probably won't be a need likely to arise for any contiguous tri-track segments (just those Worcester tri- passers, which are operationally more akin to rich-man's crossovers).
