The push pull system allows a locomotive to be driven remotely.
The driver of a train has to be at the front so as they can see where they are going. This usualy means the locomotive has to be run around the train when the train wants to change direction. If the locomotive can be remotely controlled from a cab at the other end of the train, you can avoid doing this.
It is not a new idea at all and several systems have been used for steam, diesel and electric locomotives for over a century. It has gradually come into greater use with modern diesel and electric railway traction. The electric Class 91 used on the East Coast Main Line are all push pull trains. The locomotive is always at the Scotland end of the train and the London end is a Driving Compartment and Luggage Van.
History on the FR
The first experiment with push pull operation was the Gelliwiog Shuttle, that used locomotive Moel Hebog and Carriage 110 in 1976. This allowed a train to be driven on from Dduallt up around the spiral to Gelliwiog, which is just short of the new Moelwyn Tunnel on the then incomplete Deviation. This system was based on pneumatics, and required a second man on the locomotive to change gear, as only the throttle and brakes could be remotely controlled.
In conjunction with the INCA Project and the building of Carriage 111, the diesel Conway Castle was fitted with an electronic push pull system. This allowed control of the throttle and gearbox through electrical connections and was the first true push pull as a second man was not required on the locomotive. The inclusion of a vigilance device in the locomotive and driving cab meant that the train could be operated with a single driver.
The control system of Criccieth Castle was designed from the outset with push pull working in mind and a full "Pull-By-Wire" system allows complete control and monitoring of the locomotive from the driving console in Carriage 111.
Brakes and electrical connections
Whilst the function of the locomotive is controlled by the electrical signals, the brakes of the train are vacuum operated. To allow proper control of the brakes, the driving cab needs a vacuum supply to allow the brakes to be released. This requires the provision of a second vacuum pipe along the train between the locomotive and driving compartment, and is referred to as the High Vacuum supply. Vacuum reservoirs on this supply are fitted to each carriage to improve the responsiveness of the operation of the brakes from the driving compartment. See the external link below for a description of the operation of the dual pipe vacuum brakes.
The following carriages have been fitted with the high vacuum connections and wiring for push pull operation: 110 (its driving console was removed 1988 or 1989), 111 (containing the driving console), 112, 113, 114 (Buffet Car), 117 (1977) (withdrawn 2016), 121 (1981) (withdrawn 2005) and 122.
Of these 112, 113 and 114 have lost their high vacuum reservoirs during the conversion of the heating system to diesel fired hot air heating around 2010.