What is the name of the underground rail system in Vienna, Austria?

The underground rail system in Vienna, Austria, is the Vienna U-Bahn.

The name “U-Bahn” is the German abbreviation for Untergrundbahn, meaning “underground railway.” Vienna’s modern rapid-transit system is operated by Wiener Linien and is identified by lines numbered U1 through U6, although the network currently has five active lines because U5 has not operated as a regular line in the modern system.

Modern U-Bahn service began with test operations in 1976 and officially opened on 25 February 1978. The system incorporated earlier transport infrastructure, including sections of the Vienna Stadtbahn. That history explains why not every route looks like a newly built subway: some parts run in tunnels, on elevated structures, or in open cuttings.

The Vienna Tramway is a separate street-running network, Wiener Lokalbahnen operates regional services, and the Vienna S-Bahn is a commuter-rail system. All may connect with the U-Bahn, but none is the answer to this question.