"Subway tunnels" refers to the underground passageways or channels that are specifically designed for trains or subways to travel through. Full definition
Luckily, there is an app for navigating subway tunnels all over the world. (journeytom.com)
It's hard if you take mass transit: you can't do business calls on a crowded train, you can't get a wireless signal in subway tunnels, you can't spread out your papers when you're changing buses. (thefederalist.com)
The governor in his speech also called for the MTA to study extending a subway line from lower Manhattan into Red Hook through the building of a new subway tunnel. (amny.com)