Having thought about it in the past, I think the stage would be better if the hut over the rolldown area had no roof and was just a walkway again.
As for the ending, there are 2 ways about it, the industrial and the spytech. Nuclear reactor would work, but only if there was a cooling stack just below the end of the tracks, and it is the final destination of the bombs. As for industrial, an idea I just had is a big factory with a train depot. It may seem low key, but it could have trains whizzing in and out in the valley below, and a big curved glass roof which the bombs could smash through.