The train is travelling at 40mph. If the man can escape either way then he can cover ΒΌ the distance of the tunnel by the time the train reaches the entrance to the tunnel. If running in the other direction he will thus be half way through the tunnel by the time the train reaches the entrance. Since he only escaped by the slimmest of margins, the train effectively reaches the end of the tunnel at the same time, and hence travelled the entire length of the tunnel in the time it took the man to travel half the length of the tunnel. So the train is travelling at twice the speed of the man.
