[HN Gopher] A short post on short trains
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A short post on short trains
Author : surprisetalk
Score : 39 points
Date : 2025-07-30 14:15 UTC (8 hours ago)
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| salynchnew wrote:
| Note: This only seems to apply to driverless trains.
|
| Depending on the system in questoin, this might be impractical or
| impossibly dangerous.
| jrockway wrote:
| I don't think there is any danger to running more frequent
| longer trains. Ultimately you are constrained by the signalling
| system; nobody is suggesting "just disable the tripcocks and
| plow through red signals". Modern systems use moving block cab
| signals, so are set up to succeed with high frequency. (But
| there are other problems, like terminal capacity, that can
| limit frequency. The MTA in NYC spent a ton of money giving the
| L CBTC to run 40+ tph... but the terminal at 8th Ave can't turn
| that many trains. Until that's fixed, the L will always be
| overcrowded.)
|
| You can see whether the problem is being cheap or if it's
| actually a capacity issue. If weekend service sucks but peak
| hour service is good, then it's just being cheap. If rush hour
| can't handle passenger volumes, then you need a signal system /
| automation upgrade. (Or a parallel line!)
|
| As a New Yorker, I'm very jealous of Vancouver's SkyTrain
| system. ("But NYC has the best metro in North America!!"
| Maybe...) The NYC subway has a lot of peak hour capacity. I
| hate traveling at peak hours. So that means I'm always standing
| in stations waiting 10 minutes for the next train. If I lived
| in Vancouver, then that would be 3 minutes. Sounds good to me!
|
| I also agree with the author in that I'm not sure what elevated
| trains did to hurt people. I lived in Chicago next to the L for
| years. It never bothered me. It's nice to see out the window
| and look at something while you're in transit. And it's cheaper
| than building things underground. NYC apparently got rid of its
| elevated railways because of snow, so that's something to watch
| out for. But Chicago gets more snow and it does OK. (Having
| commuted on both systems in the snow, it's a mixed bag. Chicago
| doesn't shut down, but it's slow as people remember how to deal
| with snow. NYC can run on snow-free underground tracks, but
| sometimes the governor is like "fuck it, I'm cancelling all the
| trains anyway" and then you're just stuck.)
| troupo wrote:
| > I also agree with the author in that I'm not sure what
| elevated trains did to hurt people.
|
| I've only seen them in Hamburg. They are generally an eye-
| sore. And the area underneath them is somewhat unusable (like
| any area under a bridge): you can't really build anything
| there. So it's an eyesore above an eyesore.
| PaulHoule wrote:
| Off-peak the wait for trains seems like forever in NYC.
| theluketaylor wrote:
| SkyTrain vs Seattle Link Light Rail is a fascinating
| contrast.
|
| SkyTrain works because of a virtuous cycle of attributes:
| Full grade separation enables automation. Automation enables
| many trains per hour. Many trains per hour with short trains
| still has tons of capacity. Short trains means lower costs
| for stations, which as the article notes is a huge portion of
| rapid transit costs. Lower cost means building more transit
| for the same budget, so more transit gets built. More transit
| with great headways results in transit being the mode of
| choice. Take any one of these elements out and you can still
| have a functioning transit system, but the magic is missing.
|
| Link light rail is so close to full grade separation but not
| quite there, so headways are limited by the grade crossing.
| With longer headways, bigger trains are needed to serve the
| same capacity. Bigger trains mean big stations and beefier,
| more expensive viaducts. Big stations are expensive.
|
| Link is gaining ridership and offering great service, but
| it's hard not to think if they had learned the full lessons
| from nearby Vancouver it would be even better (and cheaper).
| mschuster91 wrote:
| > I also agree with the author in that I'm not sure what
| elevated trains did to hurt people.
|
| You need to either build them in right at the start of a new
| development or you gotta demolish _a lot_ of housing -
| similarly to what was done in a lot of US cities when the
| highways were built [1].
|
| [1] https://www.reuters.com/world/us/us-freeways-flattened-
| black...
| crazygringo wrote:
| > _I also agree with the author in that I 'm not sure what
| elevated trains did to hurt people._
|
| They're horrifically ugly for pedestrians and city life
| generally. If you've been around the elevated subway tracks
| in Brooklyn, for example, they're not pleasant to be around.
| They block out the sun, they're incredibly noisy, they make
| the street claustrophobic, they definitely become streets to
| avoid unless necessary.
|
| Yes, they have a nice view if you're a _passenger_. But they
| make the city much, much worse for everyone below them.
| xnx wrote:
| > Small Train is Good Train
|
| In a few years I think we'll recognize that self-driving vans (in
| a few different sizes) are the best trains: cheaper than trains
| (and therefore lower environmental impact), completely flexible
| routing, and you can even let them use dedicated right of way if
| you want to increase capacity.
| sofixa wrote:
| > cheaper than trains (and therefore lower environmental
| impact)
|
| How does that work? For the same amount of capacity, vans are
| not and probably never will be cheaper. Steel wheels on rails,
| with massive capacity, is just drastically more efficient, even
| with 0 driver costs. You'll still have much more maintenance
| (the tires, breaks, road) to do with a much higher number of
| vehicles.
| skybrian wrote:
| This will depend on density and what's already built. An
| extreme case: it wouldn't make sense to replace a large,
| rural school district's bus fleet with trains.
| xnx wrote:
| All trains, tracks, and stations are specially designed and
| built, which makes them very expensive. There is no mass-
| produced off-the-shelf solution like with cars and roads.
|
| Trains are unbeatable for inland freight between limited
| destinations, but don't make much sense for current day
| commuting or traffic patterns.
| theluketaylor wrote:
| There are off the shelf trains, tracks, and stations
| already. It's one of the key ways many european and asian
| operators deliver tons of transit for the dollar.
| troupo wrote:
| > self-driving vans (in a few different sizes) are the best
| trains
|
| Until you actually do the basic middle-school math of
| calculating capacity. Both of vans, and of the roads
| PaulHoule wrote:
| https://en.wikipedia.org/wiki/Personal_rapid_transit
|
| seems to be the worst of all worlds but never quite dies
| thesz wrote:
| > if we run a system for smaller trains, we can build smaller
| stations for these trains, saving a huge amount on station costs.
| This costs us in reduced total capacity, but this can easily be
| made up for by increasing train frequency.
|
| There is a safe minimal distance between trains, in fact, a safe
| distance for a given speed. Shorter trains are not exempt from
| obeying it. You can make shorter trains more frequent at the
| expense of lowering traveling speed.
|
| What is the cap of throughput is due to these speed limitations
| is an exercise left for the author of the article.
| Denvercoder9 wrote:
| > What is the cap of throughput is due to these speed
| limitations is an exercise left for the author of the article.
|
| They already did that exercise:
|
| > 3-car trains running at 30-40 trains per hour (a normal peak
| frequency for automated or even some human-driven metro lines)
| reach a capacity of about 18,000 passengers per hour per
| direction, well above the expected demand of any American line
| that doesn't go through Manhattan.
| ghostly_s wrote:
| Yes, they are assuming a best-case scenario. Driverless
| systems are very expensive for reasons that have little to do
| with the cost of the driverless trains, if you're not going
| to consider those variables this kind of armchair speculation
| is a waste of everyone's time.
| decimalenough wrote:
| They aren't though? If you're building a new line, fully
| driverless is pretty much the default these days,
| especially if the line is fully underground or elevated.
|
| What _is_ incredibly expensive, though, is retrofitting a
| line designed for manual operation to run automatically
| instead.
| decimalenough wrote:
| 40 trains per hour is in fact not "normal", but extremely
| difficult. Only a few systems in the entire world operate
| more than 30 per hour.
|
| The fundamental constraint is not technology, but people and
| physics: you need to decelerate and stop, let people
| disembark and get on, accelerate and clear the platform. This
| cycle requires a bare minimum of 90 seconds, although IIRC a
| few lines in a few places like Paris and Moscow do 85 secs.
| matt-p wrote:
| Indeed, the Victoria line in London manages 36 TPH and
| we've not bothered beating it since. It's much easier to
| run 26-30TPH with slightly more carriages.
| jefftk wrote:
| SEPTA's T [1] gets up to 70 TPH and used to handle 150 TPH.
| You can do this with multiple trolleys loading/unloading on
| a platform simultaneously.
|
| (But this strategy is orthogonal to the article, because it
| requires long platforms.)
|
| [1] https://en.wikipedia.org/wiki/T_(SEPTA_Metro)
| clickety_clack wrote:
| For capacity calculations, headway is what matters. E.g. trains
| spaced 2 mins apart means that 30 trains run in an hour.
|
| It's the same with cars. A 2s headway with cars holding 1
| person each means that the maximum capacity of a highway lane
| is 1,800 people per hour, no matter how fast they go (the cars
| are further apart at higher speeds).
| Animats wrote:
| Freeway capacity is maximized around 35 MPH. Faster, and the
| greater distance between cars reduces capacity. Slower, and
| there are not enough cars per minute per lane. So the goal of
| ramp metering signals is to throttle input to keep the
| freeway speed around 35 MPH.
| mike-the-mikado wrote:
| Interesting - I have believed for many years that it was
| around 17 MPH. I felt that this tallied with my
| observations - as traffic levels increase, vehicles slow
| down (increasing total capacity) until it falls to a
| critical speed (when slowing down reduces capacity) and
| then it changes to stop/go.
|
| In my experience (on UK roads) this critical speed is
| around 17 MPH - but it might be a little different
| elsewhere.
| namibj wrote:
| Freeway capacity chokes at the limit giving rise to
| basically overpacked lanes slumping from metastability;
| systemic control with ramp meters or I66 (within the
| Washington D.C. Metro area) style real-time dynamic
| pricing lets freeways flow properly around peak capacity
| if properly implemented.
|
| There are roads that regularly suffer from acutely
| insufficient capacity in many metro areas; specifically,
| repeatedly at times _the dynamic pricing toll that would
| discourage enough people from using it to stay
| uncongested_ would overshadow the price of a rental-with-
| driver (Uber-style) during off-peak times. It's not that
| the people shouldn't get through; it's that most people
| won't need more than a backpack worth of luggage with
| them and could thus be packed 3~4 passengers for each
| driver. Splitting the toll would be the reason to do so.
|
| Unfortunately only really dynamic congestion tolls would
| really fix the concept of rush hour traffic jams. And the
| necessary surveillance system would bring severe mass
| surveillance/tracking concerns with it at least in
| central Europe.
| mitthrowaway2 wrote:
| I think you would want to keep the road just slightly less
| dense (fewer cars, higher speeds) than the density that
| maximizes throughput, because otherwise you operate at the
| edge of an instability. Any tiny local deviation in speed
| somewhere triggers a slight local decrease in throughput,
| causing bunching which further decreases throughput and
| snowballs into a traffic jam.
|
| When operating at slightly faster than max capacity, local
| slowdowns cause a local increase in throughput, allowing
| bunches to dissipate.
| wombatpm wrote:
| 35? That seems too slow. Several years ago some pranksters
| in Chicago drove side by side at 55MPH and caused a MASSIVE
| backup for miles.
| markasoftware wrote:
| I wonder if it's possible to run trains at higher speeds closer
| to each other using fixed brakes embedded near the tracks,
| similarly to how roller coasters often have mid-course brake
| runs that are only activated in emergencies when the train
| ahead unexpectedly slows or stops.
| decimalenough wrote:
| Small trains and small stations are good until they aren't, and
| then you're screwed.
|
| Singapore built the orbital Circle MRT line exactly as the author
| wants: small trains (3 carriages, vs 6-8 on other line), small
| stations to fit these trains, frequent fully automated operation.
|
| However, the line turned out to be much more popular than
| planners anticipated, with the result that at rush hour, it's
| common to have to queue just to enter the platform where you need
| to wait for multiple trains to arrive before you can squeeze in.
|
| And the tricky bit is that there is essentially nothing you can
| do now to fix it. There's a hard physical limit (around 90 secs)
| on how fast the cycle of a train arriving, people getting on and
| off, and departing can get, and retrofitting all 30+ deep
| underground stations to be larger would be an insanely expensive
| and disruptive exercise.
| theluketaylor wrote:
| That's true, but if you've kept a lid on construction costs as
| light metros tend to do, the public will embrace building new
| relief lines that bring more service to areas with moderate
| access. That adds big capacity you can then continue to grow
| into. Paris is a great example of this, they often just add
| whole new lines rather than trying to wring a bit more capacity
| from existing ones.
| kaliqt wrote:
| This makes a lot of sense to bring life to new districts
| naturally.
| bsder wrote:
| > and retrofitting all 30+ deep underground stations to be
| larger would be an insanely expensive and disruptive exercise.
|
| But before this you had no idea that there was so much demand,
| right?
|
| It's quite a lot easier to sell a huge monetary upgrade on
| something oversubscribed rather than a huge monetary outlay
| that may be a complete white elephant.
| decimalenough wrote:
| It's an order of magnitude easier and cheaper to dig out a
| 6-car platform the first time around than to expand from 3 to
| 6 when the system is already operational. And it's a one-off
| price too: if the platform is built but not used, it incurs
| essentially no operating costs to have it sit there waiting
| for the day it's needed.
| jefftk wrote:
| _> essentially nothing you can do now to fix it_
|
| You can build another line. This one was cheap and the demand
| is clearly there. And two medium capacity lines are generally
| better than one high capacity line in terms of offering more
| options to travelers.
| bgnn wrote:
| it's all about capacity requirement. If the capacity demand is
| high, you might want to build really long trains and run them as
| frequently as possible (e.g. Istanbul Marmaray)
| PaulHoule wrote:
| There's the analysis that, in the long run, operators are
| expensive which is an argument for automation or long trains.
| IIAOPSW wrote:
| 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.
| burkaman wrote:
| 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.
| kccqzy wrote:
| The author spends a lot of words talking about short trains and
| Jersey City, but didn't once mention the Hudson Bergen Light Rail
| which was at least built in recent memory? I mean other than
| driverless operation it seems to check all the boxes. And in that
| light, I really fail to see the novelty in this argument; it's
| basically the multi-decade-old argument that building light rails
| is cheaper and better than real metro or heavy rail.
| bluGill wrote:
| there is nothing new about trains - we need to stop looking for
| innovation and just build them with small incremental
| improvements. The only useful innovation since 1920 is full
| automation (though I'm going to call high speed rail
| incremental since it is mostly a lot of small improvements on
| the way - this is debatable). Even full automation was a lot of
| incremental improvements.
|
| That said, full automation is a major game changer because it
| enables short trains with high frequency which you would
| otherwise struggle to afford. Don't build light rail, build
| light metro which is a small cost more (build a metro with
| light rail) but now you can have full automation.
| frankfrank13 wrote:
| jersey and short trains makes me think of the newark air train,
| easily the worst train in the US
| bashkiddie wrote:
| Munich is a counter example: It has many lines passing through
| (almost) 11 stations. They run at maximum speed of 40 trains per
| hour. It is a traditional block signal system with each block
| being shorter than a train - so they can run trains packed into
| tighter space. Each train opens left and right doors
| simultaneously so people can board and exit at the same time.
| Boarding still takes time. It is overcrowded at rushhour and
| brakes down regularily (a train being at a station for too long
| blocks all proceeding trains). And at Oktoberfest time, the
| stations are soo crowded that boarding takes longer.
|
| Munich is planning on building a second subway route. It just
| does not have the money nor the space to build one. There has
| been discussion on building a second tunnel below the first, or
| on enabling the common train system for overflow by suburban
| trains -- by installing more signals to run more tightly packed.
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