Why can't the MTA run double-length L trains?
June 1, 2019 9:06 PM   Subscribe

I assume there's a good reason they don't do this, but I don't know it. The upcoming L non-shutdown is going to dramatically reduce total throughput because they can only run a train through the tunnel every ten minutes and they have to alternate directions. Why can't they increase throughput by stringing together longer trains, say, twenty cars instead of ten?

Possible objections:

1. The trains would become heavier and harder to pull. But all the subway cars are individually motorized, so adding more cars automatically increases the total traction power available. Unless there's some technical reason this doesn't scale?

2. The trains would become longer than the platforms. But NJ Transit does this all the time. They just tell passengers to walk forward to a car that has a door open. The MTA did the same for a while after Sandy when the 1 train was using the five-car platform at South Ferry.

2.a. The MTA has embarked on a stronger campaign now of discouraging people from moving between cars. But if they didn't want passengers walking between cars, they could pull the front half into the station, load and unload the front half, then pull forward by ten cars' length and open doors on the rear half. It would increase dwell time a bit, but twenty cars every twenty-one minutes is still way better than ten cars every twenty minutes.

What am I missing?
posted by meaty shoe puppet to Travel & Transportation around New York, NY (9 answers total) 4 users marked this as a favorite
The limiting constraint isn’t actually how many people fit per train. Because the trains are running so infrequently, many people will opt to take a bus or ferry or Lyft or simply not go. So it’s not like they’re trying to fit the same number of people in 1/2 as many trains. Instead, they’re fitting probably 1/2 as many people in 1/2 as many trains.
posted by estlin at 9:27 PM on June 1 [2 favorites]

They aren't going to advise that number of people to move between cars. At South Ferry they advised people to board any car except the last (or switch at an earlier station), not to walk between cars. A couple of people forgetting and walking forward one car on an empty train at the end of the line (which is what happened at South Ferry) is very different to dozens of people squeezing through a full L train at every stop. It's out of the question.

So, assuming they go with your 2a. I can think of two reasons. First, people would cram into the front half of the train, which disembarks first. Second, I'm not certain that opening the doors on half of the train is something the driver has the ability to do. If not, they'd have to open them one by one, i.e. pull in, open the front ten cars by pushing ten buttons. Then close them. Then pull forward. Then open the back ten cars by pushing ten different buttons. This seems slow and error prone.
posted by caek at 9:37 PM on June 1 [3 favorites]

For 2, the MTA now prohibits passing between cars except in an emergency.

For 2a, lots of fools would start passing between cars while waiting. Dangerous.

Also, it wouldn't add one minute. Think about it. The cars would have to do this at every stop. If average dwell time per stop is, say, thirty seconds (being generous) and there are 18 stops, that's adding nine minutes to the length of the journey. I'm not sure that even works within the constraints of the bottleneck, though I don't know.
posted by praemunire at 9:37 PM on June 1 [1 favorite]

A rule of thumb I've seen used in public transport traffic modelling is that demand is elastic by 0.4 to trip length, 0.8 to frequency of trains per day and just 0.2 to capacity of individual trains. So if the frequency is going down by half, they're already going to be dealing with 40% less passengers, resulting in only 20% more crowded trains. Technically doubling the capacity should result in 10% more passengers, but it also doubles your train direct operating costs, especially power and train maintenance. And if the partial shutdown is due to single-track in the tunnel (so one train can't go through before the one going from the other end emerges), every moment of track-occupancy is precious - I suspect it'd result in both slower trains and less frequency to compensate for the length and shuttling on stations, making the overall effect null except for the cost increase. (A quick back-of-the-envelope showed me that 3 extra minutes to a 20-minute trip is a 5.2% reduction in passengers, and a 3 minute increase in intervals is a 10.4% reduction in passengers, for a total negative effect on passenger numbers.)
posted by I claim sanctuary at 10:18 PM on June 1 [12 favorites]

Along with the good points above, I don't think MTA has enough train cars to do this given their delays in acquiring new rolling stock. Plus moving twice as many cars between the L and the train yards would probably add delays somewhere down the line.

(MTA ran a competition called the Genius Transit Challenge a little while back and one of the winners had the idea to lengthen trains. It wasn't really widely acclaimed; here's one critic.)
posted by ferret branca at 7:20 AM on June 2 [2 favorites]

It’s not safe and unless they’re pulling the last 10 cars into the station to allow people to board, no one is going to use them. And there’s no way to do that, because the driver has no point of reference on where to stop and probably no signal there tell them when to proceed.
posted by Automocar at 7:25 AM on June 2

unload the front half, then pull forward by ten cars' length

There is no way for a train operator to know what "ten cars' length" is outside the station. Car markers (4 cars, 6 cars, 10 cars) are located within stations. And besides, the conductor, who is responsible for passenger safety while boarding/detraining, rides in the middle of the train and looks out over the length of the platform during a station stop to make sure no one is falling or stuck in the doors etc. Where would he or she ride in your proposed idea?

Also, Newtonian physics -- yes it would take more power to pull 20 cars. It would also take more braking power and more distance to *stop*. Different train cars have different capabilities and different amounts of wear and tear -- it isn't like either a train has every part working perfectly as if brand new, or else the train is completely broken. A train that is even beginning to have brake wear will absolutely not be able to stop at any exact spot of the operator's choosing, and this is severely exacerbated by a longer train configuration. This can create a real risk of station overruns, signal overruns, and collisions. For this among other reasons, the maximum allowed train length for EMPTY trains (transfers from one train yard to another) is 12 cars. For passenger trains, the maximum is the length of the station (the only exception being the old South Ferry station as OP mentioned).

BTW, fun fact: on the M and J lines (possibly the L line as well), certain car MODELS (types) are not allowed. Most models are 60 feet long. The R-46 models used on the F, R, and a few other lines are 75 feet long and it was determined that they cannot safely negotiate the curves in some of the BMT tunnels. The corner ends of those cars would scrape the tunnel walls.
posted by RRgal at 9:44 AM on June 2 [1 favorite]

I'm not certain that opening the doors on half of the train is something the driver has the ability to do

The conductor (not the driver, he or she does not operate doors in a full-length train) does have this ability. Actually a single door panel in a single car can be opened if desired, though not from inside the conductor's cab. But there are much more pressing issues with the 20-train scenario, as I mentioned earlier.
posted by RRgal at 11:09 AM on June 2 [1 favorite]

A couple of top-of-the-head guesses - Although each car is individually propelled, would the current draw of that many cars on the same line segment be too much? And are there points in the line where two trains pass that aren't long enough for a double-length train?

But I think the first comment is most likely. They just don't think they'll need that many cars, after lots of people choose a different and less-inconvenient transportation method. I would hope they aren't making this up off-the-cuff, I'm sure in a train-saturated place like Japan the same situation has come up. They would just have to ask around for best practices.
posted by ctmf at 12:32 PM on June 2 [1 favorite]

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