how long would it take a half-mile train going one mile a minute to go through a two mile tunnel?

Answer 1

Two and a half minutes for the entire length of the train to pass through the tunnel.

Ignoring the lenght of the train for a moment, the train is travelling one mile in one minute so it will take two minutes to travel through a two mile tunnel.

But, you need to account for the fact that the train is half a mile long, the half mile of train will take a further half minute to pass through the tunnel. Thus making a total time of two and a half minutes.