Tech level: more or less contemporary, no tech that would put you in awe.

I'm trying to make realistic, driverless, mass-transit system. The first idea was that "driverless" implies that in each tram there is a bit more space because driver lost his job. Simple, problem solved.

Later, I started to think about this in more detail. The main reason why some trains are not so frequent but long is saving labour cost. So if there is no driver, they should be shorter. Thus passenger could be delighted that they waste less time waiting, while technically speaking the number of cars would be the same. And even metro stations could be shorter, as no one would try to stop their long train.

Then I started thinking about friction. Technically speaking many short train make more air friction than few long ones. So maybe the trams wouldn't be specially shorter? Or maybe realistic trams (even with dedicated lane) would stop so often that unable to achieve any speed where the friction really matters?

OK, the question is: does anyone have any detailed calculation showing how much such technical issues like length of tram and its speed matter for energy efficiency? (Or any idea idea how to make such adjustments for driverless trams not based on gut feeling but some more or less hard data?)

(No, no individual pods, too expensive and fancy for my setting.)

  • 2
    $\begingroup$ Automated trains already exist. The question appears to conflate a bunch of questions about public transit that don't have much to do with each other, narrow it down to one? $\endgroup$
    – Schwern
    Mar 5, 2017 at 21:48
  • $\begingroup$ more frequency also means that you need more locomotives, too, because more of them are used at the same time. $\endgroup$
    – SJuan76
    Mar 5, 2017 at 23:51
  • $\begingroup$ What exactly you are asking about? You seem to use "trains", "metro" and "trams" pretty much interchangeably. Trams use tracks laid on streets. Metro has separate tracks that are not supposed to be accessible otherwise and are not supposed to cross streets (running either underground, or on elevated tracks, above surface). Metro set can be referred to as train, but apart from that train usually runs on ground, between cities and occasionally crosses roads. Either of those can be automated, but each has different purpose and different considerations. $\endgroup$
    – M i ech
    Mar 6, 2017 at 1:56
  • $\begingroup$ I'm about to jump on plane, so no time to elaborate. Check out usaprt.org for engineering details of personal rapid transit under consideration in my hometown. Driverless, with 2-person and 4-person units. $\endgroup$
    – SRM
    Mar 6, 2017 at 3:17
  • $\begingroup$ Automating a highway crossing is a nightmare. I mean, it's no problem if you don't hit anything... but if you do, then you get into a huge diversity of exceedingly rare use-cases - hard to code for, hard to test. That's not the half of it - then you have to start moving again after being stopped blocking the crossing and people being unpredictable. runningmagazine.ca/… $\endgroup$ Mar 6, 2017 at 7:24

5 Answers 5


This already exists

As an urban transportation nerd (side note, go follow Urban Planning Stack Exchange!) this is a big deal, and is the future of transit. The thing is, automated trains already exist and already work.

The Vancouver SkyTrain has 79.5 km of track and 53 stations. It has been in operation since 1985 and is fully automatic. For comparison, the Chicago 'L' system has 165 km of track and 145 stations; so Vancouver's system isn't tiny on the scale of things.

Automated trains are possible, work well, and are the future of mass transit. Go tell your local council-member, MP, or senator today.

There are many other systems out there, in addition to Vancouver's; via Wikipedia. That article talks about levels of operation: some subways are more automatic than others.

To answer the question more exactly, there are no differences in operating characteristics from human driven to automated trains. For example, the Red line in Washington DC last year shifted from human driven to Type II automation: computer driven with a human operator on board. There was no change in schedule, train length or train frequency.

There is no reason to want to shorten the trains. Trains do not have a relative advantage over cars until population and job density gets very high. There are only a few cities in the US for which an urban rail network makes sense: New York, Chicago, Boston, San Francsico, New York, Philadelphia, and Los Angeles. In other cities, the transit system would make sense only if efforts were made to increase either the population density in the downtown areas (Houston, Atlanta) or the jobs density (Dallas, Miami). Some cities have good downtown density, but just a bit too small (Minneapolis, Seattle, Denver).

Since urban rail is only an advantage to move large numbers of people in places where there is not enough room for cars, rail should concentrate on building to at least half capacity. The highest frequency subways in the world can get up to almost a train a minute through the busiest stations along the same line. NYC's 4/5/6 on the East Side runs about 49 trains, 8 cars long, per hour along the length of Manhattan during AM peak hours. Headways (time between trains at a station) are the limiting factor here.

If a rail system can't fill the tracks, then it can't make the money to pay for those tracks. Rail has high upfront capital costs and relatively high capital maintenance costs. If you drop cars from your trains, you are still limited on headways, even on automated systems. That means you are just dropping capacity and revenue.

The advantage of automated trains is in the labor costs. As I pointed out in the comments, for the MBTA in Boston, The FY15 Budget (slide 3) for was \$1.9 Billion, of which \$740 million, or about 39% was Labor costs. Based on the Boston Globe's breakdown of MBTA employees, around 40% of overall employees (bus and train) were drivers. Eliminating drivers could reasonably be expected to cut 20-30% off of operating costs for the trains. This would allow the shifting of more money to maintenance and expansion, and reduce the government's need to invest in the system, which would, of course, increase its political appeal.

Automated heavy rail the future of dense urban transit for cost based reasons. That challenge is to convince society, especially in America, to live in increasingly dense conditions to take advantage of this heavy rail future. If we can, there will be significant economic, equality and environmental advantages.

  • $\begingroup$ Also the Docklands Light Railway in London, with 38km of track and 45 stations. $\endgroup$
    – Mike Scott
    Mar 5, 2017 at 21:09
  • $\begingroup$ Copenhagen Metro. en.wikipedia.org/wiki/Copenhagen_Metro $\endgroup$
    – MichaelK
    Mar 5, 2017 at 21:11
  • $\begingroup$ Without following your links I still don't know if these are shorter, or not. $\endgroup$
    – Mołot
    Mar 5, 2017 at 23:17
  • $\begingroup$ @Molot I don't understand your question. $\endgroup$
    – kingledion
    Mar 5, 2017 at 23:40
  • 2
    $\begingroup$ @kingledion This doesn't answer the actual question. Do the automated trains use fewer cars per train and more frequent trips? Or not? How does the automation affect how the trains operate? $\endgroup$
    – Brythan
    Mar 6, 2017 at 4:35

As with anything, there are pros and cons.

There is not a significant difference in friction between the cars and the rails - each car weighs the same regardless of whether cars are adjacent to it and each car is supported on its own wheels. Shorter trains will experience more air resistance, because the cars at the beginning and end of the train (where they either plow through the air, or suck it along behind them) contribute the most to air drag.

However, the real issue is traffic. Rail cars are definitely not immune to traffic. In my city (Denver) it is quite typical, especially during peak times, for light rail trains to either have congestion among themselves at junctions, or interact with automobile traffic. For safety reasons, no more than one train at a time can be in any particular station or section of track, and so replacing four one-car "trains" with one four-car train will effectively quadruple the capacity of the railway. Similarly, a four-car train doesn't take much longer to pass through an intersection shared with automobiles than a one-car train does, since most of the time is spent waiting for traffic signals rather than actually rolling through the intersection.

Overall, bundling the train cars together makes the interaction with automobile traffic or other rail cars much more efficient. While future technologies might narrow the safety margin somewhat, one limiting factor will always be space on the platform. Since light rail systems are built in dense urban environments, it's not often practical to build parallel loading platforms, and only one train at a time will ever be able to use the platform. Longer trains will always have this advantage over shorter ones.

What you might eventually see are longer trains during peak times, and shorter ones during off-peak times. This already happens of course, but it might happen to a greater degree, allowing greater frequency of service during off-peak times (compared to how it is now).

  • $\begingroup$ The traffic issue is not a constant, it depends on the type of light rail that you have. For Instance Londons DLR runs on dedicated tracks and has no interaction with other vehicles. $\endgroup$
    – Sarriesfan
    Mar 6, 2017 at 7:06
  • $\begingroup$ MUNI did a very good job deconflicting trains, by coupling together (or reverse, separating) up to 4 cars going the same direction. "3 cars. J N N. 4 cars. K M L L." This happened where routes converged at Duboce or West Portal. You don't see it anymore, apparently it doesn't play well with the new red/silver cars. $\endgroup$ Mar 6, 2017 at 7:35

Transportation Research Board is a good place to start looking for hard numbers.

Driverless vehicles can indeed run more frequently. That frequency is goverened by the limits of the busiest segment of the line. It doesn't matter if the infrastructure at Outlying Station A can support a train every 90 seconds. What matters is Downtown Transfer Station B, serving trains from all three branches, where the dwell time might be 60 seconds, and most cars are 80% full. That location determines the capacity of the entire line.

Labor costs are important, but are roughly comparable to energy costs. Other costs include maintenance, capital costs of vehicle replacement, insurance/liability/safety compliance, security, fare-handling, and administration.

There are cost tradeoffs - more frequent automated vehicles will travel more miles each year, requiring more maintenance and more frequent replacement. Guideways will wear faster, requiring more frequent shutdowns for repair and replacement. Additional infrastructure costs like more frequent inspections, sensors, and a small cadre of programmers may be needed, reducing (perhaps eliminating!) the labor cost savings. It's likely that more frequent vehicles will attract more riders, which increases revenue...but more riders also means more security, more station and car cleaners, and associated incremental costs, too.

Increased ridership also makes that critical maintenance more challenging, shrinking work windows to middle of the night, and raising their cost. You can shut down an entire line for a day or a week...but there will be a different type of price to pay.

The question dealt a lot with energy questions, so here are some energy answers:

In general, most vehicles (like trams) have negligible air resistance below about 30 mph...but air resistance is a power curve, and climbs quickly above that. City trams can be boxy because they are slow. Slow vehicles lose much more energy to motor efficiency losses and transmission losses than to air resistance.

Above 30mph, subway and commuter trains are longer, and do benefit from some energy savings, though that's usually incidental. These large-infrastructure systems often have expensive legacy constraints like platform length, siding locations/lengths, network branches, shared traffic, and signalling systems that make many changes unfeasible without years of planning and investment.

Both maintenance costs and energy costs are also affected by the weight of the vehicles. Lighter vehicles pound the guideway less, and require less energy to move...but lighter materials are often less durable (fiberglass) or too expensive (carbonfiber). Materials must be resistant to the inevitable casual vandalism by riders...which is why stainless steel and plexiglass, though heavy, remain common.

  • $\begingroup$ Or other parts of the infrastructure -- lightweight transit railcars, even if standard gauge, may not be able to withstand being humped or kicked during shipment, for instance $\endgroup$
    – Shalvenay
    Mar 7, 2017 at 0:47

What I envision is an automated car that will come or stop for you on demand; larger cargo pods are also available. You get in and tell it where you want to go.

These start out and finish up as singles, but link up into trains to aggregate traffic sharing a route. They would also automatically load into encapsulators to make use of different modes of travel; e.g. pods are normally efficient local electric cars, but link into a train pulled by a powerful engine when taking a freeway a longer disance, or load onto what are essentially rail flats to take a light rail system; likewise for boats or air travel.

On a rail-only system like you are asking, will it be a fixed route loop, or allow choosing a destination? Let me make an analogy with elevators: new systems more than double capacity and reduce waiting for each passenger by having you state a destination in advance and then taking the indicated cab. A fixed loop rail, ideally with pulloffs for stations, could have the same benefit.

You get in a car that goes where you plan to get off. Groups going the same way for any distance will link up on the fly, and unlink when one wants to go a different way at a junction.

If tracks are shared among different possible routes, you need a minimum space between trains for safe switching. But in general, a group travelling on the same segment that are close together will link up on the fly.


Subway car frequency and speed - which it seems you are attempting to optimize for - is quite definitely not limited by cost of the labour of the 'drivers'. It is limited mostly by the fact that you want to run a schedule and you need to have some wiggle room when someone blocks the door or something, because you don't want the next subway car to have to break like crazy every time that happens. The result of this is that practically the highest frequency turns out to be about one subway car every two minutes (whether you look at driverless rail systems like the SkyTrain or countless of normal subways all over the world) and making them shorter is not going to make that any better (if anything the added complexity would just make it worse).

Bonus: Morgantown Personal Rapid Transit system

If you just want a creative alternative driver-less system (with definite advantages and disadvantages) than this multi-lane solution is worth a look at. Every station has its own 'off ramp' and every 'car' goes straight from start to destination without stopping at stations in between. Turns out that in real life it's not really worth it, but it's quite an easy sell in when world building.

Two cars passing as they move in opposite directions

The Morgantown system uses automated control and operates in three modes, "demand", "schedule", and "circulation".[15]

Demand mode operates during off-peak hours and reacts dynamically to rider requests. After pressing the button to call a car, a timer starts. If the timer reaches a predetermined limit, typically 5 minutes, a vehicle is activated to service the request even if no other passengers have requested the same destination. Also, if the number of passengers waiting to travel to the same destination exceeds a predetermined limit, usually 15, a vehicle is immediately activated.[15] In this mode the system operates as a true PRT.

During peak hours, the system switches to schedule mode, which operates the cars on fixed routes of known demand. This lowers the waiting time for a car traveling to a given destination and is more efficient than demand mode. During low-demand periods, the system switches to circulation mode, operating a small number of vehicles that stop at every station, like a bus service. This reduces the number of vehicles traveling on the network.[15]

Source: https://en.wikipedia.org/wiki/Morgantown_Personal_Rapid_Transit

And here is a nice video showing it https://www.youtube.com/watch?v=iaSaWfw07Sw


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