The U-Bahn Line B is the second line in the network of the Frankfurt U-Bahn. It leads in west–east direction from the central railway station (Hauptbahnhof) over the old town to Konstablerwache, where it splits into two branches to Bornheim (U4 service) and Preungesheim (U5 service). Originally planned as an independent main line, the D line represents an extension of the U4 to Bockenheimer Warte from the central railway station.

The northern branch to Preungesheim was opened in 1974 and was used by the B1 service, and since May 1978 by U5. A large part of the route consists of a former tram line, which runs in Eckenheimer highway on street-level railway track. The platforms were also very low on the parts of the track, meaning that only the converted Ptb trams could run on the line. From 2013 to 2016, the above ground stations were developed barrier-free. Since 9 October 2016 the subway cars type U5 run on the U5 service.

Inaugurated in 1980, the U4 runs between Bockenheim and Bornheim exclusively on independent railway tracks and in tunnels. In 2008, the line was extended beyond the existing terminus Seckbacher road on existing tracks by the depot East to Schäfflestraße in the district Riederwald and drove for the first time also partially above ground. In December 2008, this trial operation went into regular operation, with the line extended to Enkheim. Every second to third train of the U4 goes there on a special track in the course of the Borsigallee, the existing above-ground tracks of the C line together with the U7 service. Since then, the U4, like the other Frankfurt underground services, has not been independent of individual traffic on the entire route.