The A4 pacifics 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 dynamometer 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. 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 spotters as "streaks".