The number of tubes in a tunnel is the number of parallel passageways in it, which are separated by barriers along their full height (but not necessarily along their full length). Usually the different tubes are connected at various points for safety.

The separating barrier distinguishes one tube from the next. Connections between tubes do not prevent them from being distinct parallel passageways.

The number of tubes is different from the number of vehicle lanes or rail tracks. The count concerns the enclosed passageways rather than the transport paths within them.