The biggest challenge is identifying said solid object. It is only within the last five years that cars with radar have been able to detect stationary objects with some level of reliability, prior to that, if it wasn't moving, there is a good chance that your Adaptive Cruise Control wouldn't detect it, as otherwise they had a tendency to stop for the bin that is on the side of the road, or for parked cars.

A train is a particularly challenging problem, since it is moving at high speed, even with two cameras it is card to really figure out how far away it is. Just because it is easy for humans, doesn't mean it is easy for computers, and vice-versa.