p.s. How did you get on with the pivot thingy for the shadows?
Haven't had a chance to try it yet, but I will at some point.
I'm sure you've thought of all these things, and most are probably unrelated, but I'll chuck around some ideas just in case -
1. What sort of collision do you have set for the pivots? Is it the sliding type? Perhaps the pivots are being pushed into the ground or into each other which is increasing the collision time.
2. Have you tried replacing them with spheres so you can observe what they do?
3. what happens if you give the edges of the rails a slightly off-vertical surface, ie. angle both edges inwards so the rail top is narrower. Perhaps the 90degree angle at the base/ground is causing problems.
4. are the pivots colliding with anything else by accident, perhaps the wheels or vehicle body or something is inadvertantly set.
5. how big is the mesh? Perhaps it's a size problem - could you scale the whole world down somehow to test it?
6. is the ground plane one large poly? Again perhaps a size issue so maybe breaking it into lots of smaller polys would help.
7. perhaps try a resetentity on each pivot to clear the collisions each frame?
8. Perhaps a huge number of collisions are being generated - have you tried debugging a countcollisions() to see how many are taking place?