"Rollbocks, sometimes called transporter trailers, are narrow gauge railway trucks or bogies that allow a standard gauge wagon to ´piggyback´ on a narrow-gauge line. The Vevey system enables a coupled train of standard gauge wagons to be automatically loaded or rolled onto Rollbocks, so that the train can then continue through a change of gauge.
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." - (en.wikipedia.org 12.04.2021)
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