The system uses a pair of narrow gauge (750 or 1,000 mm) rails laid in a pit that is built in the middle of a standard gauge track, which is elevated by about 30 cm. It allows the "Rollbock" bogies to sit underneath the standard gauge tracks and as the "Rollbock" train is pulled out of the "Rollbock" siding each bogie picks up one axle of a standard gauge wagon as it rises out of the "Rollbock" pit. Thus two "Rollböcke" are needed for a twin-axle wagon. They were a development of the transporter wagon ("Rollwagen"), designed to keep cost and weight down by avoiding the need for a complete wagon.