Question 1094814
<br>It takes 10 seconds for the train to pass a stationary point, like the ceiling light.  So it also takes 10 seconds for the whole train to pass the entrance to the tunnel.<br>
Since it is 30 seconds from the time the train first enters the tunnel until it is completely through the tunnel, and since it takes 10 of those seconds for the end of the train to reach the beginning of the tunnel, there are 20 seconds left for the end of the train to reach the end of the tunnel.<br>
Then, since it takes 10 seconds for the train to pass a stationary point, in those 20 seconds the train travels twice its length.<br>
So twice the train's length is equal to the length of the tunnel, which is 400m; so the length of the train is 200m.