The slipping of a coach (or coaches) by detaching from a moving train began in the late 19th century. It was practised by a number of UK railways but abandoned by all but the Great Western, in the early years of the 20th century. The GWR went on performing slips for 12 years into the BR era, ending in 1960.
A slip coach enabled a fast service to an intermediate station without the extra time imposed by stopping the whole train at the platform. This offered great advantages to passengers, both those who had a fast journey to a station that was not a main stopping point and for those on the main train who were not delayed by intermediate stops.
The down-side of slip coaches was that their passengers had no access to the rest of the train for refreshments etc, and that there was no means of attaching a coach in the opposite direction without stopping the train. Slip coaches had to be worked forward to a convenient starting point for their return, usually by attachment to local stopping services.
The system required a guard, travelling in the slip coach to have control of the special coupling and the brakes. The slip coach was fitted with a coupling hook on which the upper section was hinged. The coupling of the rear coach of the main train was placed over the slip coach hook, which was locked in place by a wedge. The wedge was moved by operating the slip lever in the coach, allowing the hinged part of the hook to open and release the coupling. Self-closing seals were fitted to the brake pipes and these operated as soon as the hoses parted.
The slip coach was fitted with large vacuum reservoirs – cylinders on the underfame, or on the roof of some early slip coaches. These allowed the slip guard to apply and release the brakes as necessary to stop the coach in the right place once it was detached.
Though, at some locations, slip coaches were detached at the approach to a station platform, in most instances slipping took place on a through line and the coach was collected by the station pilot and moved to the platform. This was certainly true of locations such as Westbury, where the slip would take place on the through cut-off route, and the coach would need to be moved round to the station. It was not normal practice to switch points between the passing of the main train and that of the slip coach where slipping took place close to
