If the motor in the railtruck can draw more current than an LM-3S can provide (for example, 4 Amps), then you couldn't use an LM-3S to drive the motor directly. What can be done (in theory) is to use the LM-3 to drive a transistor designed for delivering a lot of power, or a pair of them in a configuration called a darlington pair and the transistor(s) then provides the power to the motor, easily supplying more than the LM-3 can deliver on it's own. Something very similar is already happening inside the LM, there is no way the microcontroller inside the LM can deliver 1A by itself.
Unfortunately I don't have a good drawing to post to explain it, nor do I have an explanation of how to handle forward and reverse. Our resident electronics guru would have been able to but sadly Alan has not been here for quite a long time for some reason.
- Tim