OK, another plug for Miku Miku Dance:

Take a look at this video

[flash=425,344:15sbrqiy]http://www.youtube.com/v/DU08c8h6a2U[/flash:15sbrqiy]

This was made using the open source software "Miku Miku Dance", which is able to import SketchUp models. The trains, I think, are actually driven by the rotation of their wheels, and a "Camera Follow" effect is used to keep the view on one of the trains.

Something like this can be made using SketchyPhysics, I'm sure.