2 2/3 miles. The trains are exactly 4 miles apart. Their
combined speed is 15 miles/hour, so it will take them 16 minutes to collide. The
first train will have travelled 1 1/3 miles, so 1/3 mile of it will be out of
the tunnel in front, & 2/3 mile of it will be out in the back on collision.
The other mile of it will be in the tunnel. The second train will have travelled
2 2/3 miles, so only 1/3 mile of it will still be in the tunnel, so 1 2/3 miles
of it will be out of the tunnel.