Electro-pneumatic brake system on British railway trains

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Introduction

The Southern Region of British Railways operated an intensive self-contained fleet of electric multiple units for suburban and middle distance passenger trains. From 1950 an expansion of the fleet was undertaken, and the new build adopted a braking system that was novel in the UK, the electro-pneumatic brake in which compressed air brake operation was controlled electrically by the driver. This was a considerable and successful technical advance, enabling a quicker and more sensitive response to the driver’s operation of brake controls.

Origins

From the 1920’s, the Southern Railway of the UK and its predecessor companies had adopted electrification and multiple-unit train operation as a solution for dense and intensive passenger service requirements. The fleet prior to World War 2 used the two-pipe Westinghouse Air Brake system, which was more effective than the generally prevailing vacuum brake then in favour in the UK. However it had disadvantages, chiefly:

  • Partial release of a Westinghouse brake application was unresponsive, and usually required a full release – which took a considerable time – and then a re-application
  • On a long train the brake force during a brake application was not consistent along the length of the train; the response to the driver’s operation of the brake valve varied according to train length, and the variation caused longitudinal surging
  • Release after a full application is slow
  • Response to the driver’s operation of the brake valve was inconsistent and not self-lapping (that is, the position of the brake control valve set the rate of change of brake force, not the level of the brake force).

The First EP-fitted units

Starting in 1950 a large new fleet of suburban multiple units were delivered to a pre-war design, and as well as other technical improvements they were equipped with the electro-pneumatic brake – universally referred to as “the EP brake”. The advance in braking technology dominated the other developments, and the designation of the train units was 2-EPB and 4-EPB for the two- and four-car units respectively. The design was successful, and a larger fleet of broadly similar design was built, and the Kent Coast electrification extended the adoption of the EP brake to medium distance passenger operation, but still confined to multiple units. (A small fleet of locomotives were built for the Southern Region and had a brake control system fitted that was compatible for interoperability purposes.)

Westinghouse and EP brake operation

The trains had Westinghouse brake equipment, and had an electric control system, activating the compressed air brakes on each coach. In normal operation, the driver used the EP system exclusively, but it was not fail-safe. If the electrical system failed, the driver merely had to move the brake valve to a further position and the same valve operated the fail-safe Westinghouse system on the train. This was only done in case of failure or emergency.

The Westinghouse system uses air reservoirs on each vehicle and compressed air is released from these reservoirs to the brake cylinders as the pressure in the train pipe is reduced by the driver operating the brake valve. This process causes a mechanical linkage to press the brake blocks against the wheels.

The release of compressed air into the brake cylinders is achieved by triple valves, which are themselves controlled by the pressure of air in the train pipe, a pneumatic pipe running the length of the train. When the driver wishes to make a brake application, he operates the driver’s brake valve which releases some air from the train pipe, so operating the triple valves. When the driver wishes to release the brakes, his operation of the driver’s brake valve returns compressed air (stored in a cylinder near the driving position) into the train pipe and this moves the triple valve so as to release the air in the brake cylinders to atmosphere, allowing the brake blocks to move clear of the wheels. Although in multiple-unit operation the air reservoirs on the vehicles can be relatively quickly charged, the restoration of pressure to the train pipe takes some time as air has to travel physically down the length of the train.

In EP operation a distributor, performing functions similar to those of the triple valve, is operated directly and instantly by electrical control from the driver’s brake valve. The Westinghouse brake cylinders and air reservoirs and pumps are used, so that only the means of transmitting the driver’s command is changed.

Advantages and Developments

The advantages of the EP system are that:

  • The driver’s brake valve is self-lapping; the position of the valve activates a specific brake pressure in the brake cylinders, and therefore a specific braking rate
  • The distributors are activated instantly and simultaneously, so that there is no longitudinal surging, and the response is consistent irrespective of train length
  • Release of a brake application starts instantly in response to the driver’s brake valve, throughout the train; and partial release and re-application is possible.

The electrical control system required control cables throughout the length of the train (in addition to the two air pipes for Westinghouse operation) and a jumper cable was provided at each end of every unit for use when two or more units were coupled to run in multiple. In 1950 no electronic control was possible, and the system used four conductors in the cable to achieve graduated braking rates.

The EP brake system was greatly appreciated by drivers and was adopted for subsequent builds of rolling stock on the Southern Region of British Railways, including middle-distance passenger stock. However it is useless if any vehicle in the train is not equipped with it, and its adoption was limited to the self-contained Southern Region activity, and a small number of other isolated electric operations which used modifications of the same fleet. British Railways later adopted developments of the EP brake, but later multiplexing and now digital control systems are all but universal for passenger operation in the UK. However, a very small fleet of later medium-distance units has been retained to operate the Brockenhurst to Lymington branch, and the braking system on these units is substantially the original EP system.

Further Reading

Moody G T, Southern Electric 1909-1979, 1979, Ian Allan Ltd, ISBN 0 7110 0924 4
"Electro-pneumatic brakes" on Railway Technical Web Pages gives a world view in context at [1]

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