 It allows the "Rollbock" bogies to go 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 costs down by avoiding the need for a complete wagon.