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.
a) Simplest: One entered at 7am and the other at 7pm, so they entered 12 hours apart
b) Trickier: The tunnel straddled a time zone boundary, so the trains entered one hour apart