I am curious to know how many railroad buffs know what keeps the wheels of a train on the tracks. The first answer that comes to mind, which is wrong, are the flanges. The flanges on the wheels actually are not necessary, and are only there as an insurance policy for engineers who take corners a little faster than they probably should. The physics of a railroad wheel is actually more subtle. If you look at the cross-section of a wheel on a track, you will notice that the wheel does not sit flat on the track, but is beveled. And the bevel increases dramatically as the tread approaches the flange. The reason for the bevel is that it changes the radius of the wheel. For the physics to work, the wheels on an axle must be fixed and not be free to rotate independently. As a wheel rides up to "jump" the track, the larger radius of the bevel causes it to rotate slower (large wheels rotate slower than small wheels at the same speed). Consequently, the wheel on the opposite rail moves such that it contacts the rail at a smaller radius. It would like to rotate faster, but it can't because the wheels aren't free to rotate independently. It's beautifully complicated, but the bottom line is that it forces the truck to turn inward, centering it on both rails. Try putting a large wheel and a small wheel on the same axle and watch how it rolls. This process actually is continuous and automatic, so the wheels will stay centered on the rails, unless Casey Jones is going too fast. Then the squealing sound of rail on flanges will be heard, which, by the way, is nicely built into the Trainz program.
Jim
Jim