I am wrapping up a project to look at speedup possibilities for trains between New York and New Haven; I’ll post a full account soon, but the headline result is that express trains can get between Grand Central and New Haven in a little more than an hour on legacy track. In this calculation I looked at speed zones imposed by the curves on the line. The biggest possible speedups involve speed limits that are not geometric – and those in turn come from some very sharp slow zones. The worst is the Grand Central station throat, and I want to discuss that in particular since fixing the slowest zones usually yields the most benefits for train travel times.
Best practice for terminal approaches
Following Richard Mlynarik’s attempt to rescue the Downtown Extension in San Francisco, I’ve assumed that trains can approach terminals at 70 km/h, based on German standards. At this speed, an EMU on level track can stop in about 150 meters. In Paris, the excellent Carto Metro site details speed limits, and at most terminals with bumper tracks the speed limit is 60 km/h, with a few going up to 100 km/h.
Even with bumper tracks, 70 km/h can be supported, provided the train is not intended to stop right at the bumpers. At a fixed speed, the deceleration distance is the inverse of the deceleration rate. There is some variation in braking performance, but it’s in a fairly narrow range; on subway trains in New York, everything is supposed to brake at the same nominal rate of 3 mph/s, or 1.3 m/s^2, and when trains brake more slowly it’s because of a deliberate decision to avoid wearing the brakes out. As long as the train stops 1-2 car lengths away from the bumpers, as is routine on Metro-North, the variation will be much smaller than the margin of safety.
Fast movement through the station throat is critical for several reasons. First, as I’ll explain below, sharp speed limits have an outsize effect on trip times, and can be fixed without expensive curve easements or top-rate rolling stock. And second, at train stations with a limited number of tracks, the station throat is the real limiting factor to capacity, since trains would be moving in and out frequently, and if they move too slowly, they’ll conflict. With its 60 km/h throat, Saint-Lazare on the RER E turns 16 trains per hour at the peak on only four tracks.
I had a conversation with other members of TransitMatters in Boston yesterday, in which we discussed work to be done for our regional rail project. One of the other members, I forget who, asked me, do European train protection systems shut down in station throats too?
The answer to the question is so obviously yes that I wanted to understand why anyone would ask it. Apparently, the American mandate for automatic train protection on all passenger rail lines, under the name positive train control, or PTC, is only at speeds higher than 10 miles per hour. At 10 mph or less train operators can drive the train by sight, and no signaling is required, which is why occasionally trains overrun the bumpers even on PTC-equipped lines if the driver has sleep apnea.
Without video, nobody could see the facial expressions I was making. I believe my exact words were “…What? No! What? What the hell?”.
The conversation was about South Station, but the same situation occurs at Grand Central. Right-of-way geometry is good for decent station approach speed – there is practically no limit at Grand Central except tunnel clearances, which should be good for 100 km/h, and at South Station the sharp curve into the station from the west is still good for around 70 km/h given enough superelevation.
The impact of slow zones near stations
Last year, I published code for figuring out acceleration penalties based on prescribed train characteristics. The relevant parameters for Metro-North’s M8 is initial acceleration = 0.9 m/s^2, power/weight = 12 kW/t. Both of these figures are about two-thirds as high as what modern European EMUs are capable of, but it turns out that at low speed it does not matter too much.
Right now the Grand Central throat has a 10 mph speed limit starting just north of 59th Street, just south of milepoint 1. The total travel time over this stretch is 6 minutes, a familiar slog to every regular Metro-North rider; overall, the schedule between Grand Central and Harlem-125th Street is 10 minutes northbound and 12-13 minutes southbound, the difference coming from schedule padding. The remaining 65 or so blocks are taken at 60 mph, nearly 100 km/h, and take around 4 minutes.
Now, let’s eliminate the slow zone. Let trains keep cruising at 100 km/h until they hit the closer-in parts of the throat, say the last kilometer, where the interlocking grows in complexity and upgrading the switches may be difficult; in the last kilometer, let trains run at 70 km/h. The total travel time in the last mile now shrinks to a minute, and the total travel time between Grand Central and Harlem shrinks to 5 minutes and change. Passengers have gained 5 minutes based on literally the last mile.
For the same reason, the Baltimore and Potomac Tunnel imposes a serious speed limit – currently 30 mph through the tunnel, lasting about 2 miles; removing this limit would cut 2-2.5 minutes from the trip time, less than Grand Central’s 5 because the speed limit isn’t as wretched.
The total travel time between New York and New Haven on Metro-North today is about 1:50 off-peak, on trains making all stops north of Stamford. My proposed schedule has trains making the same stops plus New Rochelle doing the trip in 1:23. Out of the 27-28 minutes saved, 5 come from the Grand Central throat, the others coming from higher speed limits on the rest of the route as well as reduced schedule padding; lifting the blanket 75 mph speed limit in Connecticut is only worth about 3 minutes on a train making all stops north of Stamford, and even on an express train it’s only worth about 6 minutes over a 73 kilometer stretch.
What matters for high-speed travel
High-speed rail programs like to boast about their top speeds. But in reality, the difference between a vanilla 300 km/h train and a top of the line 360 km/h only adds up to a minute every 30 kilometers, exclusive of acceleration time. Increasing top speed is still worth it on lines with long stretches of full-speed travel, such as the Tohoku Shinkansen, where there are plans to run trains at 360 over hundreds of kilometers once the connection to Hokkaido reaches Sapporo. However, ultimately, all this extra spending on electricity and noise abatement only yields a second-order improvement to trip times.
In contrast, the slow segments offer tremendous opportunity if they are fixed. The 10 mph limit in the immediate Penn Station throat slows trains down by around 2 minutes, and those of Grand Central and South Station slow trains by more. A 130 km/h slog through suburbia where 200 km/h is possible costs a minute for every 6.2 km, which easily adds up to 5 minutes in a large city region like Tokyo. An individual switch that imposes an undue speed limit can meaningfully slow the schedule, which is why the HSR networks of the world invented high-speed turnouts.
Richard Mlynarik notes that in Germany, the fastest single end-to-end intercity rail line used to be Berlin-Hamburg, a legacy line limited to 230 km/h, where trains averaged about 190 km/h when Berlin Hauptbahnhof opened (they’ve since been slowed and now average 160). Trains go at full speed for the entire way between Berlin and Hamburg, whereas slow urban approaches reduce the average speed of nominally 300 km/h Frankfurt-Cologne to about 180, and numerous compromises reduce that of the nominally 300 km/h Berlin-Munich line to 160; even today, trains from Berlin to Hamburg are a hair faster than trains to Munich because the Berlin-Hamburg line’s speed is more consistent.
The same logic applies to all travel, and not just high-speed rail. The most important part of a regional railway to speed up is the slowest station throats, followed by slow urban approaches if they prove to be a problem. The most important part of a subway to speed up is individual slow zones at stations or sharp curves that are not properly superelevated. The most important part of a bus trip to speed up is the most congested city center segment.