Douglas Harper's Etymology Dictionary
German or Austrian subway system, 1938 (originally in reference to Berlin), from German U-bahn, short for Untergrund-bahn, literally "underground railway."
Usage examples of "u-bahn".
It was to this sprawling Prussian fortress, opposite the busiest U-bahn station in the city, that March went next.
Buhler's old colleague from the General Government, Schongarth, has slipped beneath the wheels of a U-bahn train in Zoo Station.
I phoned everyone from the Berlin Documents Centre to Onkel Toms Hutte U-bahn Station.
I bought a copy of Time and a Daily Express at the U-bahn station across the road and sat down in Kranzlers at a table from which I could see the whole width of the Kurfiirstendamm as far as the Gedachtniskirche.