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New Zealand doesn't have particularly advanced technology for its railroads, but I have seen pickup trucks that have wheels that can run on railroad tracks as well as a separate set of wheels with tyres for driving on the road. I think they are used for light maintenance, and probably for quick access to hard-to-reach places.

What is the fuel efficiency of a vehicle that size when it operates on the railroad tracks?

If there are similarly sized vehicles that only operate on tracks, then they would obviously be lighter and thus more efficient (and cheaper!), so some data on that kind of vehicle would be of interest too (if it exists!).

Edit: The Aramis personal rapid transit project in France (from 1970 to 1987) appears to have tried to use vehicles similar to what I'm describing. There appear to be some other projects listed here.

  • What is the motivation behind the question? Are you interested in the long-term possibility of replacing a significant number of roads with rails, or the marginal efficiency of driving one more truck with special wheels on existing tracks? – Nate Jun 29 '13 at 22:48
  • @Nate, I guess I'm wondering why Google (or someone) isn't trying to develop car-sized automated on rails when it's likely to be so much easier than, for example, trying to develop an automated car that drives on roads (like Google is doing in California). – Highly Irregular Jun 30 '13 at 5:00
  • My understanding is that their autonomous car project is aiming to facilitate technology that will allow normal cars on normal roads to drive without user intervention ... which would save massive amounts of fuel, reduce traffic, and reduce accidents, if it's done correctly. And, also artificial intelligence is cool, and Google probably sees it as good publicity. Sadly, trains are not perceived as being cool, or "high-tech" in the US :( – Nate Jun 30 '13 at 5:58
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    @Nate - I think it is more that cars are much more convenient than trains that run late on limited schedules and are often no station is in short walking distance of our destination. It is not a coolness factor. – user141 Jul 2 '13 at 13:07
9

From the general principles outlined in Sustainable Energy--without the hot air by David J.C. MacKay (http://www.withouthotair.com/c20/page_118.shtml):

  1. In short-distance travel with lots of starting and stopping, the energy mainly goes into speeding up the vehicle and its contents. Key strategies for consuming less in this sort of transportation are therefore to weigh less, and to go further between stops. Regenerative braking, which captures energy when slowing down, may help too. In addition, it helps to move slower, and to move less.
  2. In long-distance travel at steady speed, by train or automobile, most of the energy goes into making air swirl around, because you only have to accelerate the vehicle once. The key strategies for consuming less in this sort of transportation are therefore to move slower, and to move less, and to use long, thin vehicles.
  3. In all forms of travel, there’s an energy-conversion chain, which takes energy in some sort of fuel and uses some of it to push the vehicle forwards. Inevitably this energy chain has inefficiencies. In a standard fossil-fuel car, for example, only 25% is used for pushing, and roughly 75% of the energy is lost in making the engine and radiator hot. So a final strategy for consuming less energy is to make the energy-conversion chain more efficient.

These observations lead us to six principles of vehicle design and vehicle use for more-efficient surface transport:
a) reduce the frontal area per person;
b) reduce the vehicle’s weight per person;
c) when traveling, go at a steady speed and avoid using brakes;
d) travel more slowly;
e) travel less;
and
f) make the energy chain more efficient.

We’ll now discuss a variety of ways to apply these principles."

It seems that a pickup truck operating on railroad tracks would get a benefit from traveling at a steady speed for longer distances, but would still be at a disadvantage compared to trains in that:

  1. It is not long and thin (does not pull more people or freight in its wake, after it has already done the work of moving the air in front of it).

  2. Its drive train (internal combustion engine) is not efficient as a train's electric drive (powered by a diesel generator, but still more efficient).

There is also an advantage of using railroad wheels on rails instead of rubber on concrete. According to http://en.wikipedia.org/wiki/Rolling_resistance, the railroad wheels have a coefficient of rolling resistance of 0.0010 to 0.0024; while car tires on concrete get 0.010 to 0.015. The resistive force is proportional to this coefficient, and various websites list rolling resistance as about 5% of the energy loss in on-road vehicles, so by my calculations, this aspect of running a truck on rails rather than on the road might save you 4-5%.

  • Welcome, and thanks for this. Do you have a secondary source - perhaps one that specifically addresses the question of steel-on-rail vs rubber-on-road friction? I've found that MacKay's book is unreliable in many places. – EnergyNumbers Jul 2 '13 at 3:18
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    According to en.wikipedia.org/wiki/…, the railroad wheels have a coefficient of rolling resistance of 0.0010 to 0.0024; while car tires on concrete get 0.010 to 0.015. The resistive force is proportional to this coefficient, and various websites list rolling resistance as about 5% of the energy loss in on-road vehicles, so by my calculations, this aspect of running a truck on rails rather than on the road might save you 4-5%. – Elaine Hale Jul 2 '13 at 15:00
  • Good stuff - could you add that to the answer - you should be able to edit it straight in – EnergyNumbers Jul 2 '13 at 20:20

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