Advanced collisions: mesh collision?

Hi all.

This is somehow related to my previous question about trains and rail tracks. Now I have some rail tracks and train on it. Rail tracks have Box collider collider set. The train itself has Box collider too. But it is too general model. I want more precise collision detection, in a way that train car wheels collide with tracks, so I can implement something like train derailment or more advanced physics events. Can you suggest a possible way to improve collision detection? Exactly: how can I make a collider that detects a collision between each rail and train car's wheels on it?

Thanks.