[divingdragon]

> Crashes would be avoided by having every train know about the train ahead and behind, and unable to make any move which would cause a collision (ie. it is not allowed to slam the brakes on if there is a train right behind you).

You are assuming that a train will never have to suddenly stop. This will never fly in the real world. Even if you consider a completely closed railway system with no possibility of external obstructions, there are many kinds of failure that would cause modern trains to apply emergency breaks due to fail-safe designs.

If you remove the bit about not allowing to slam on the breaks, then you just described SelTrac. Even the first version used on the Vancouver SkyTrain (opened in the 80s) is capable of running trains closer than braking distance from what I remember reading. I don't believe it is actually enabled on many SelTrac systems though, because you still need to have safety margins. There is always the possibility that the train in front may decelerate at a rate higher than its emergency braking rate, like if it derailed or collided with external obstructions.