Sir Nigel Gresley introduced the famous A4 class locomotive in 1935 to pull a new train called the Silver Jubilee, between London King's Cross and Newcastle, in celebration of King George V's 25th year of reign.
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".