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The A4 pacifics (with the 4-6-2 wheel arrangement) were designed for low consumption of coal and water on all kinds of services; passenger and freight. With the introduction of the double-exhaust Kylchap blastpipe , the consumption levels of the above dropped even more, gaining more revenue to their operators.
On July 3 1938 the Mallard, newly fitted with the Kylchap exhaust, set a world speed record of 125 mph (201.2 km/h), pulling six cars plus a dynamometerA dynamometer or dyno for short is a device used to measure power and torque produced by an engine. There are two types of dynos; One that gets bolted directly to an engine, known as an engine dyno, or a dyno that can measure power and torque without remo car. Although the dynamometer car indicated a top speed of 126 mph (202.8 km/h), Sir Nigel Gresley never accepted this speed as the record-breaking maximum. He claimed this speed could only have been attained over a few yards (metres). He was comfortable that the German speed record of 124.5 mph (200.4 km/h) had been surpassed.
The A4 class locomotives were known to train spottersTrain spotting is a pastime practised largely in the United Kingdom and the United States (and elsewhere) in which enthusiasts spend time at railway depots, train stations or at trackside vantage points noting down serial numbers of passing trains and loc as "streaks".