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):
- 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.
- 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.
- 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;
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:
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).
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%.