The most famous tunnel is the thirty mile "Chunnel" between England and France. The Chunnel is actually two separate tunnels, connected by cross passages.
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 female wood mouse has multiple mates, so the sperm of this wood mouse may be competing with sperm from other males.