What is "rollbock"

Rollbocks, sometimes called transporter trailers, are pre-coupled narrow gauge transporter trucks or bogies that allow a coupled train of standard gauge wagons to be automatically loaded or rolled onto so that the train can then continue through a change of gauge.

The system uses a narrow gauge rail running in a pit that is built in the middle of a standard gauge track. 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.