> So if we have three trains in close succession and the first derails or loses power, the second is not allowed stop because there's a train behind it?

The second is allowed to follow the 'train ahead derailed' plan, which presumably will involve stopping at a rate that the third train can also achieve.

If comms is maintained during the event, new plans can also be made that are maybe better (eg. maybe try to brake harder than the guaranteed minimum braking)