Comment by IIAOPSW
7 days ago
Something I've been thinking about lately is short trains but long platforms. Basically, you split the length of the trains in half and then split the platform into two boarding areas. In this way trains can be scheduled even closer together than the the 3-min physical limit of of signalling because the one coming in behind doesn't need to wait for the one ahead to leave the station first. Its therefore possible to schedule several different services on the same two-track system such that they all skip a large fraction of stops (and thus run faster), but every station is still reachable from every other without transferring.
A simple example would be 3 services on the same line that follow a repeating pattern every 3 stations. ==[A]==[B]==[C]==[A]==[B]==[C]==. The service (AB) stops on the stations labelled [A] and [B] (skipping C). The service (AC) stops on the stations labelled [A] and [C] (skipping B). The service labelled (BC) stops on the stations labelled [B] and [C] (skipping A). In this manner, all three services skip over 33% of the stops on the line, but no matter what your origin and destination, it is always possible to travel from an [A] station to a [B] or [C] station (or another [A] station), and likewise from a [B] or [C] station.
To the extent I've worked out the logistics of this, if you allow for trains to catch up but not pass each other at the platforms, you can push this idea as far as only stopping at 2-in-5 stations without sacrificing headways or capacity.
Just a weird thing thats been taking my attention lately.
In a lot of train systems this exists in the form of fast / stop trains, the fast trains only do the bigger train stations, the stop trains stop at every station, servicing smaller stops.
As for short trains on long platforms, this is pretty common in NL where the bigger train stations can support both very long international trains and shorter local trains on the same platform; a train can switch to a center track halfway on the platform.
I don't think it would really work for a subway system; people expect it to be hop on, hop off. In some places the stops (or people's final destinations) are so close together they can choose to get on/off earlier/later, but this system makes that less viable. You'd have more people shuffling through train stations trying to figure out which train to get or whether they need to wait for the next one, also putting extra load on staff for confused not-locals. And finally, you'd need extra rails or tunnels so that a train can pass another.
It can work on a subway system because there are skip-stop service patterns on J/Z line in New York.
In terms of needing extra track, that's the brilliant part of it. Even though the different services all make different stops in my proposal, so long as they depart in a certain order they will never need to pass each other.
In the 2-in-3 stops example I gave, suppose that at the present moment there's an (AB) train at station [A], a (BC) train at station [B], and a (AC) train at station [C].
If they all depart at the same time then (AB) goes one stop to the next [B] at the same time as (BC) goes one stop to the next [C] and (AC) goes one stop to the next [A].
Then they should all depart again at roughly the same time (since they all presumably took the same amount of time to go one stop).
So (AB) goes two stops from [B] to the next [A] station at the same time as (BC) goes two stops from [C] to the next [B] station and (AC) goes two stops from [A] to the next [C] station.
Then the cycle repeats.
All three service run at the same time without ever catching up or passing each other.
If you use my two-boarding-area trick then the constraint can be loosened to allow different services to catch up (but not pass) each other, which then makes it possible to run 2-in-5 stops without adding a passing track.
To elaborate a bit, on the face of it the 2-in-5 system has 10 different services running on the line at the same time, because there's 10 possible unique combinations of 2 out of 5 things.
However, just like in the 2-in-3 example, many of these services can be run at the same time without passing or catching up because they are scheduled to move and stop in the same pattern (just offset from a different starting station.
Every single train in the 2-in-5 system can be categorized as either moving 1 stop then 4 stops, or moving 2 stops then 3 stops. Two services in the same category never catch up or pass each other. If you have two subsequent services in a different category then they simply take turns catching up and falling back behind (but ultimately never passing each other).
Here's a (rough) line diagram I calculated showing where each train is at every 3-min interval in the 2-in-5 system. Notice how they meet but never pass.
https://ibb.co/4R9nQsQd
In terms of confusing people and knowing which train to get on, this is what I've come up with as a signage convention. All that's necessary to know what train to get on is the symbol on the map of your destination. If a train has the same symbol on it then it goes there.
https://ibb.co/tpjzJdyR
I feel as if the time spent by each passenger walking to the right boarding area could easily surpass the dwell time of an extra stop or two.
Consider a metro system with trains 70 m long. With 10 m of space between the two boarding areas, that means the length between both sides of the station is 150 m. If the entrance to the platform is in the center, the walk to the middle of each boarding area is 45 m, taking about 30 seconds. If the entrance is at the end, that becomes 35 m or 115 m (taking about 20 to 90 seconds to walk). I think those figures are comparable to the dwell time of a typical metro system.
I do think it's a very interesting idea though! I think it'd work better for longer trains over longer distances, where the time spent accelerating and braking is often greater, but unfortunately few places are even considering anything shorter than 15 minute headways for such rail services.
Admittedly my calculations did not take walking-on-platform time into account, but what I have been assuming (based on figures I've looked up) is that each stop adds about 60 seconds to the journey time (30 seconds for dwell + 15 on either end for getting up to / down from cruise speed). I've also been assuming that the inter-station no-slowing-down time is 2 min.
So in a 2-of-5 stopping pattern you'd save 3 min every 5 stops, but depending on when arrive at the station you will sometimes get unlucky and just miss your train, in which case you'll have to wait an additional interval than you normally would for the next one (costing you 3 min). So for very short trips (under 3 stops) it actually makes travel time a bit worse since there's very few stops you can skip over in such cases. But for more median trips there is a saving, and for the longer trips it saves well into 2-digit minutes.
In your example I think in the case with the entrance at the middle of the platform there wouldn't be any real effect, because splitting the platform also in the middle doesn't marginally change how quickly you can get to the nearest open door on the train you want. There's no real need to get to the center of either boarding area. In the case of platform at the end of the station, I can accept that adds about 40 m of walking distance that wasn't there before (at least for some people), and that translates into an extra 20ish seconds. Less than that if you do the rational thing of walking briskly when you see you're about to miss your train.
So you're right that the platform split adds some time which isn't completely negligible, and depending on what fraction of the next n stops you want to skip it comes at a penalty of longer wait times for the correct train, but both of these are fixed upfront costs whereas the dividend accrues linearly the longer your on the train.
How would that help? 3 minutes is the minimum separation distance, if the trains have different stopping patterns then that just means they'll be further apart for part of their journey. If you have to stay 3 minutes behind the train in front, you might as well stop at all the same stations that it does, you can only save time if you were further behind to start with, so you can't increase throughput by skipping stops.
Simplifying a bit, its because the train behind can be scheduled 3 min after the train ahead has pulled in rather than 3 min after the train ahead pulled out. Put another way, the safe stopping distance to maintain (while at speed) would be measured from the half way point of the platform rather than the start of the platform.
This lets you schedule them much closer together than the conventional 3 min while still being safe.
> its because the train behind can be scheduled 3 min after the train ahead has pulled in rather than 3 min after the train ahead pulled out.
Why? If the minimum is 3 min then it's 3 min (at least with a modern moving-block type setup). If it's safe to run them 2 min apart it's (generally) safe to run them 2 min apart the whole time.
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This is how it already works in Europe (at least in Austria and Germany, but I assume elsewhere in Europe as well).
You will also find long trains that split in half mid-journey, so you need to make sure you get in the right car or you'll go to the wrong place.
Edit: I guess it's not exactly what you're saying, in Europe you will find platforms split into several sections with multiple trains to board, but they'll be for different lines with different destinations.
yeah, what I have in mind is a variation / generalization of skip-stop.
https://en.wikipedia.org/wiki/Skip-stop
The main cost of construction is station length, so making longer platforms with shorter trains is the worst of both worlds.