This is similar to the pole and barn paradox, but different enough that I'm not quite confident in my answer: A train and tunnel have the same length when at rest relative to each other. The train approaches the tunnel with constant velocity. An engineer on the train places rockets on the front and rear, equipped with timing devices such that the rockets will be launched simultaneously, in a vertical direction, when the midpoint of the train passes the midpoint of the tunnel. Do the rockets cause any damage to the tunnel? My thought here is that "timing devices" is key. It seems to mean that the engineer somehow calculates at what time he will pass the midpoint, as opposed to sending a signal to the rockets when that happens. In any case, the obvious answer is that since the tunnel is contracted in the train's frame, the rockets don't hit the tunnel. That also must mean that the rear rocket it launched first in the tunnel frame--before the midpoint is reached. Is that right? It doesn't seem right to me if I consider a timing device on the train. Say it's set to go off at t=10 (calculated by the engineer to correspond with the midpoint passing) and it's started when the front of the train passes the front of the tunnel. In the tunnel frame, the timer runs slow, so if the rocket goes off at some time before the rear enters the tunnel, it has gone off before the timer reaches 10. What am I missing here?