You probably visit a gas station every few days, but have you ever paid attention to octane numbers?
When a train enters a tunnel, it compresses the air in front of it like a piston. Air in a tunnel can't be simply pushed aside--the tunnel walls are in the way.
The most famous tunnel is the thirty mile “Chunnel” between England and France. Thirty miles of air is a lot to push around, so engineers have come up with a clever solution. The Chunnel is actually two separate tunnels, connected by cross passages.