Push Pull
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 in to 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.
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.
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 conpartment, and is referred to as the High Vacuum supply.
All of the carriages built as part of the INCA Project were fitted with the high vacuum and electrical connections, allowing the full six carriages of C set to be used as a push pull train.
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.
Currently the following carriages have been fitted with the high vacuum connections and wiring for push pull operation: 110, 111 (containing the driving console), 112, 113, 114 (Buffet Car), 117 and 122.
Carriage 121 (Withdrawn 2005) was also fitted for use with the push-pull.
