It runs between Farum and Høje Taastrup and provides stopping services on the S-train system's Farum radia and Tåstrup radial.

[1] Since the first part of the Taastrup radial opened in 1953, letter B has been used for its principal service.

Before that the characteristic of service B was that it was the stopping trains to Holte.

In 1979 a separate daytime reinforcement service Bb was created; it ran every 20 minutes with a 10-minute offset to service B such that the Tåstrup radial effectively had a 10-minute frequency.

Under the timetable doctrine followed in those years, a service letter such as B could not be used for more than exactly 3 trains an hour, so a separate service designation was needed for these daytime supplements.