The Berlin U-Bahn uses 750-volt DC electric trains that run on standard gauge (1,435mm) tracks. The first trains were based on trams and take their power from an upward facing third rail.
To accommodate greater passenger numbers without lengthening the trains, trains that ran on lines built after World War I were required to be wider. The original trains and lines continued to operate, designated as Kleinprofil (small profile), the newer, wider trains and lines were designated Großprofil (large profile). Großprofil trains are 2.65m wide and take their power from a downward facing third rail. This is related to New York City’s A Division and B Division systems, where the B Division trains are wider than A Division trains.