You might not be able to fit a full-sized train in there, but don't forget the original New York City Subway had to fit in some relatively tiny tunnels: http://www.nycsubway.org/wiki/The_Construction_of_the_Harlem...

Subway trains are often engineered to specification, so if you have a tunnel of a particular size the manufacturer can make a train for it.

The precursor to the modern subway was a strange pneumatic system that would fit in that tunnel as well: http://www.nycsubway.org/wiki/Beach_Pneumatic_Transit