Demonstration Video: http://www.youtube.com/watch?v=gssAMZ_J1lI
First, make a curved track. And put wall on outer side, and make celling so track fit in 1x1 area. Now put cart on track, and push it toward from inner track. Cart will explode as soon as it rotates curve, and destroys supporting rail. Cart also explodes when I drop it from high height, but I havent figured out how to reproduce them. Maybe TNT cart code is related to Anvil; since it does not 'explode' when dropped from low, but does from high.