2 Nov 2012

The cost of speed

A train running at 125 mph consumes 90% more energy than one running at 90 mph. In addition, there are other costs, since the trains have to be specified for higher speeds, one-third of the front and rear vehicles cannot be used for passenger accommodation, there is additional wear and tear, signalling systems must be designed for the longer stopping distances, thereby reducing track capacity, and the railway becomes subject to EU regulations for high speed lines, with all the associated compliance costs.

Reams of careful calculations would need to be made before the proposal to reduce top speed on some 125mph routes to just under the 100 mph threshold could be dismissed as lunatic.


  1. Nice! If anyone object to your suggestion you just ask them for their reams of calculations.

    Simply though any trip where the train runs at 95mph rather than 125mph (assuming the rest of the times are the same) for a distance of about 35 miles means a journey time increase of 5 minutes.

    The other thing that is worth pointing out is that much of the UK's intercity lines and rolling stock are already designed to run at 125mph, meaning that many of the "costs" are already taken on board.

    1. You have taken top speeds as average speeds, haven't you? How much of the 35 miles is actually run at top speed depends on acceleration and braking rates. On the GW, for instance, trains between London and Bristol nearly all stop at Reading, and many stop at several others of the following: Slough, Didcot, Swindon, Chippenham, Bath. And on the Westbury route there is no 125 mph running anyway, nor is there much west of Exeter. Unless the train has a scorching rate of acceleration, it will be a good way down the line before it gets up to top speed.

      Even though the system is currently designed and constructed for 125mph running, the additional costs of running the services and maintaining the infrastructure and rolling stock for running at that speed are considerable. What is the real value of the time saved?

      There was work done on journey time elasticity in the 1970s but to what extent does it apply now? Similar calculations are applied with respect to light rail, which leads to top speeds of not more than about 80kph, again there is a threshold above which higher speeds give rise to significant extra costs.

      Interestingly the ECML has never been upgraded for 140mph running although the mark 4 stock was designed to run at the higher speed.

  2. I said "Simply though any trip where the train runs at 95mph rather than 125mph for a distance of about 35 miles means a journey time increase of 5 minutes."

    I did not say any journey between two stations 35 miles apart.

    The big cost saving with faster trains is that it allows you to run more services with the same number of trains. For instance a train with a journey time of 56 minutes from end to end can turn around and run the service back and be ready to run the service again in 2 hours time.

    While if it takes 61 minutes end to end (even allowing no turn around time) it misses being able to do so by two minutes and so requires another train to run this service. One extra train may not make much difference, but over a large enough network and you could be looking at quite a lot of extra trains.

    1. Thanks for your reply and interest. As you say, there can be benefits, and savings in trains and staff is one of them, and other things being equal, people prefer a shorter journey. And there are threshold times that make for good marketing. However, whether these make it easier to operate a clockface timetable is another matter. The Brighton fasts now take 49 minutes which gives an 11 minute turnround, a bit tight for comfort but it usually works. When the journey took 55 minutes, as you point out, more trains had to be provided on the route.

      But, and it is a big but. Southern services tend to run at slower speeds than other main lines. As a result of this lower specification, Electrostars and Desiros were about a million pounds a vehicle when the high speed Pendolinos were double. Other costs are in proportion. What you get on Southern routes is a regular interval walk-on service with a simple fares structure to fill up the off-peak trains and discourage people to crowd on at the busiest times. You turn up at the station, buy your ticket and go. The trains don't go much above 90 mph but they are reasonably comfortable so you can get on with doing something.

      North of London it is another storey. Partly because of the higher cost of the rolling stock and the entire operation, good yield management is essential for profitability. So either you pay a hefty standard fare or take advantage of the advance booking offers. But they tie you to a particular train. So to be sure of catching the train you have paid to travel on, you allow plenty of time for the journey, usually turn up at Euston with half an hour or more to spare and hang around waiting to get on your train, even though you were actually in time for the one before! The paradoxical result is that your door-to-door journey takes longer than the slower walk-on service which is too expensive to provide owing to the higher costs of the higher speed operation.

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  4. A train moving at 125mph (56m/s) uses 67% more energy than a train at 90mph (40m/s). - that's the energy required for movement - as calculated using the Davis formula for the Swedish X2 train. See "Comparing the Environmental Impact of Conventional and High-Speed Rail" (Network Rail) data from UIC EVENT project 2003.

    However 20% of energy consumption comes from "train services" (ie read air-conditioning etc) slightly reducing this figure further. (I find this figure surprisingly high)

    If you attempt a simple calculation of energy per km consumption (ignoring the "train services" correction) you get 1.67 / (125/90) = ~1.2 .. that is 20% more energy at 125mph per km travelled compared to 90mph, all other things being equal.

    The UIC report can be downloaded from http://www.uic.org/download.php/environnement/energy_EVENT.pdf - it goes into more detail with a wider analysis

    Your claim that "signalling systems must be designed for the longer stopping distances, thereby reducing track capacity" is one-sided - and does not take into account higher track utilisation from higher speeds - what is the real world effect?

  5. ..more..
    It's worth noting that running at 90mph with peaks of 125mph is not a bad approximation of what UK high speed trains do currently - in many respects your proposal actually represents current operating practice. (slightly less but not by that much)

    I leave it to you to do the energy sums for the actual HS2 proposal, which includes running at up to 250mph...

  6. re "one-third of the front and rear vehicles cannot be used for passenger accommodation" - not sure where this impression came from - it is true that UK pendolinos use the forward driving ends for storage/services etc - but that is intended as a feature, not a 'bug'.

    Modern HSTs can and do have full seating in driving cars - eg AGV, ICE3, European Pendolinos etc.