I would like to share with you a construction for a roller coaster animation that I discovered to be very useful.
It is based on a mesh that rides on a path due to a curve modifier. Every three vertices of this mesh are a parent for a cube (that represents a single car of the coaster).
The train will follow the path in every direction / twist, but the single cars will not deform. The train’s speed is controlled by a loc-IPO-curve.
You can get a .blend file with the basic construction in the following thread of the German blender forum blend.polis:
(It is the very last file I uploaded named “Roller_coaster_300907”
And you can watch a rendered animation demonstrating the effect on youtube:
Hope you find this helpful for your own roller coaster or other path animations.