A train enters a tunnel at 7 o'clock. Another train enters the exact same tunnel at the other end, also at 7 o'clock on the same day.
The tunnel has only one track, no passing places and no other means for the trains to pass, around, under or over.
However, both trains made it safely through the tunnel.
How?
The first train entered the tunnel at 7:00 AM, and the second train entered the tunnel at 7 PM or, vice-versa.