Limitations
The progress represented by the automatic vacuum brake nonetheless carried some limitations; chief among these were:
- the practical limit on the degree of vacuum attainable means that a very large brake piston and cylinder are required to generate the force necessary on the brake blocks; when a proportion of the British ordinary wagon fleet was fitted with vacuum brakes in the 1950's, the physical dimensions of the brake cylinder prevented the wagons from operating in some private sidings that had tight clearances;
- for the same reason, on a very long train, a considerable volume of air has to be admitted to the train pipe to make a full brake application, and a considerable volume has to be exhausted to release the brake (if for example a signal at danger is suddenly lowered and the driver requires to resume speed); while the air is traveling along the train pipe, the brake pistons at the head of the train have responded to the brake application or release, but those at the tail will respond much later, leading to undesirable longitudinal forces in the train. In extreme cases this has led to breaking couplings and causing the train to divide.
- the existence of vacuum in the train pipe can cause debris to be sucked in. An accident took place near Ilford in the 1950's, due to inadequate braking effort in the train. A rolled newspaper was discovered in the train pipe, effectively isolating the rear part of the train from the driver's control. The blockage should have been detected if a proper brake continuity test had been carried out before the train started its journey.
A development introduced in the 1950's was the direct admission valve, fitted to every brake cylinder. These valves responded to a rise in train pipe pressure as the brake was applied, and admitted atmospheric air directly to the underside of the brake cylinder.
American and continental European practice had long favoured compressed air brake systems, the leading pattern being a proprietary Westinghouse system. This has a number of advantages, including smaller brake cylinders (because higher air pressure could be used) and a somewhat more responsive braking effort. However, the system requires an air pump. On steam engines this was usually a reciprocating steam pump, which was quite bulky. Its distinctive shape and the characteristic puffing sound when the brake is released (as the train pipe has to be recharged with air) make steam locomotives fitted with the Westinghouse brake unmistakable.
In the UK, the Great Eastern Railway, the North Eastern Railway, the London Brighton and South Coast Railway and the Caledonian Railway adopted the Westinghouse system. It was also standard on the Isle of Wight rail system. This led to compatibility problems in exchanging traffic with other lines. It was possible to provide through pipes for the braking system not fitted to any particular vehicle so that it could run in a train using the "other" system, allowing through control of the fitted vehicles behind it, but with no braking effort of its own.
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