There is going to be major investment in the Northeast Corridor, and several possibilities, including Amtrak’s NEC Master Plan, call for running trains at higher frequency and somewhat higher speeds than today on the Providence Line, and assumes electrification of commuter service. Since the line is already being used by the MBTA, which according to Amtrak is limiting the number of intercity train slots for capacity reasons, this calls for a good measure of schedule integration, based on the principle of organization before electronics before concrete.
Amtrak’s Master Plan calls for three-tracking the entire Providence Line south to Attleboro (one viaduct excepted) instead, at a cost of $464 million – $80 million in Phase 1, $384 million in Phase 2 – in addition to money spent on unnecessary expansion at South Station. Such a cost is excessive, suggesting that better MBTA-HSR compatibility is required. Full-fat HSR programs go even further and avoid the Providence Line in favor of a greenfield alignment or an I-90 alignment, instead of making use of the existing high-speed track in Rhode Island and Massachusetts. To reduce costs, a better plan would four-track short segments for passing sidings, and time the overtakes. The principle is similar to that of the blended Peninsula plan in California, in the version proposed by Clem Tillier.
In many ways, for example the metro area populations involved and the current ridership level, the Providence Line is similar to the Caltrain line. The main difference is that the Providence Line has fewer stops and therefore can expect higher average speeds. In addition, the Providence Line is straighter and passes through less developed areas, so that even today Acela trains plow it at 240 km/h, and about 330 km/h is possible with true high-speed trains and higher superelevation.
In Switzerland, trains run as fast as necessary, not as fast as possible. In this context, this means running just fast enough to meet a good clockface schedule. Boston-Providence travel time on the MBTA today is about 1:10; for a good takt, this should be cut to about 55 minutes, allowing hourly service with two trainsets and half-hourly service with four.
For the purposes of schedule symmetry and avoiding switching moves at high speed, passing segments should have four tracks rather than three when possible. Costs should be controlled by making those passing segments much shorter than the three-tracking Amtrak proposes.
Finally, the timetables proposed here are based on the following performance assumptions: regional trains have a top speed of 160 km/h, accelerate like a FLIRT (45 seconds acceleration plus deceleration penalty), have an equivalent cant of 300 mm, and dwell at stations for 30 seconds. Intercity trains accelerate like an idealized N700-I, have an equivalent cant of 375 mm, and dwell for 60 seconds. The equivalent cant is by and large unimportant; the acceleration and dwell times for regional trains are. The approach into and out of South Station has a speed limit of 70 km/h through the 90-degree curve toward Back Bay, and 100 km/h to south of the curve at Back Bay; intercity trains are limited to 200 km/h south to Readville and 250 km/h south to the Canton viaduct, and, at the southern end, 225 km/h west of the curve in Attleboro and, curves permitting, 200 km/h in Rhode Island. Regional trains turn in 5 minutes, or 4 at a minimum, and intercity trains turn in 10 minutes at a minimum. Signaling allows a headway of 2 minutes at a speed of 200 km/h and 3 minutes at higher speed, but if a regional train starts from a siding stop, it can follow a high-speed train more tightly initially, say 1 minute, still far higher than a safe stopping distance, since the spacing rapidly increases over time. Grades are ignored; the Providence Line is flat enough that they’re not an issue. Timetables should be padded 7% from the technical time.
With the above assumptions, the technical time for regional trains is 38 minutes with the present stopping pattern, which yields 41 minutes with padding; this compares with 46 minutes for the fastest Acela. Clearly, if Acela service levels remain similar to what they are today – which includes the Master Plan, which calls for a 10% reduction in Boston-New York travel time (see page 40 on the PDF linked above) – there’s no need for passing segments. To raise travel time to 55 minutes, trains should make more frequent stops, and/or run to T. F. Green Airport always. Although the speed profile of regional and intercity trains would be different, the average speed would be the same, and given that the corridor has a small number of trains per hour of each type, this mismatch is no cause for concern. The $464 million Amtrak is proposing would then be a complete waste, and the federal government should spend any money toward this goal on electrifying more MBTA lines and funding EMUs.
However, in a scenario involving a significantly improved intercity service, the best technical time for nonstop Boston-Providence service with a top speed of 300 km/h decreases to about 19 minutes (20.5 with pad), and this makes overtakes necessary. A slowdown to 250 km/h only adds about one minute of travel time, so the operating pattern is almost identical.
If 15-minute service, both regional and high-speed, is desired, then regional trains can be about 11 minutes slower between successive passing segments, since 11 = 15-3-1 or 15-2-2. A single mid-line overtake is theoretically possible: 41-20.5 = 20.5 < 2*11. However, such an overtake would have to be exactly at the midline, and, in addition, there could be merge conflicts at Providence, whose station tracks include two on the mainline and two on one side of the mainline as opposed to one on each side.
It’s still possible, but tight, to have a single overtake at Sharon. The immediate station vicinity would be four-tracked; this is no trouble, since the area around the station is undeveloped and reasonably flat. In addition, there’s more than enough time in the Providence area, making the merge conflict a lesser problem. However, this is very tight near Boston South, beyond signaling capability unless four-tracking extends a few kilometers further north. One way to counter this problem is to slow high-speed trains by making them all stop at Back Bay and/or Route 128, adding precious minutes to the schedule but reducing the speed difference. Conversely, the current weekday pattern of Providence Line trains skipping Ruggles could be made permanent. There is no room for infill stops; the overtake would only add 4 minutes to regional train travel time, so there’s time to run further to the airport at 160 km/h, and even make an extra stop at Cranston.
Another possibility is to have two overtakes, taking advantage of existing four-tracking around Attleboro. The capital costs are similar; it would require four-tracking around Route 128, possibly extending north to Readville if an on-the-fly overtake is desired. The operating complexity is much higher, since there’s one more opportunity for a late train to mess up the entire schedule. However, there is plenty of slack south of Attleboro and north of Route 128 allowing for additional stops. Under this option, the train loses 4 minutes waiting at Attleboro and about 2.5 at Readville, since the overtake is not completely on-the-fly, raising travel time to 47.5 minutes. There’s no time for airport trains, not on the same takt. However, there’s space in the schedule for 5-6 infill stops in addition to Readville; Forest Hills, Pawtucket, Central Falls, and perhaps one more in each of Boston and Providence closer to city center.
In principle, it’s possible to extend this analysis to 10-minute service, with three overtake segments, at Route 128, Sharon, and Attleboro. In practice, this is operationally cumbersome, and the operating profits coming from filling six full-length high-speed trains from New York to Boston ought to be able to pay for four-tracking the entire line, even the viaduct.
Not included in this analysis are the branches. Those are not a worry since north of Readville there are three tracks, and frequencies on the other lines are low. The Stoughton Line is a bigger problem; however, with the three tracks through Boston, it could still be shoehorned. Electrifying it should not be difficult due to its short length, though the proposed Taunton extension would make it harder.