tube train

Etymology
Probably named after the shape of the tunnels; the is an informal name for  which has been trademarked.

Noun

 * 1)  A smaller-profile underground train which operates through the tube tunnels under London, which have a small cross section.

Usage notes
Tube trains are operated on the Bakerloo, Central, Jubilee, Northern, Piccadilly, Victoria and Waterloo & City Lines. The trains that operate on the Circle, District, Hammersmith & City and Metropolitan Lines pass through larger tunnels and accordingly have a larger profile; these should be called underground trains, not "tube trains".