The Rollbock system allows a coupled train of standard gauge wagons to be automatically loaded or rolled onto pre-coupled narrow gauge transporter trucks or bogies so that the train can then continue through a change of gauge. The Rollbock bogies 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.
Also known as the Langbein system, this method enables the Rollbock wagons to traverse curves as sharp as 15 m (49.2 ft) radius and, when fully loaded, they could be moved over narrow gauge tracks at a safe speed of 13 mph/21 km/h.[1]
They are used extensively in Switzerland and in Spain, in the latter country to transport standard gauge vehicles on broad gauge lines.
They are also still in use at Nordhausen on the Harz Narrow Gauge Railways to transport limestone wagons from the narrow gauge to the DB system.
Until the 1990s the Rollbock sidings at Wernigerode were used every day to transport goods to and from the various metallurgical factories attached to the Harz Narrow Gauge Railways in the immediate area. Similarly it was used extensively on the Saxon narrow gauge system west of Dresden to transport china clay to Meissen.
However, it means that the narrower gauge network must be built to a structure gauge large enough to accommodate the loading gauge of standard gauge wagons, negating one of the cost advantages of a narrower gauge construction. One could of course use standard gauge wagons especially built to a smaller than normal loading gauge.