The Mallard record was a bit of a stretch. One way, down a hill, not in service but testing new brakes. It needed repairs after that. The previous record was 1.5 mph slower.
At least it didn't have the wheel slip that the 4-4-4-4 PRR train had or that poppet valve problem.
Size was also very different, like comparing a 737 to a 747.
Beautiful that the Mallard was, the U.S. trains had something to them and the expanse of the U.S. gave opportunities that were not possible in the UK for some truly fast trains. The mountain ranges of the US are vast compared to the mere hills of the UK, if you wanted a downhill run for top speed then the U.S. should have had options. But planes came along.
edit: Btw. taken from the german wikipedia, and put into DeepL because I'm lazy:
After several test runs with trains with a mass of about 250 t and speeds of up to 195.7 km/h, the 05 002 reached a world speed record for steam locomotives on May 11, 1936, in front of a train made up of four cars (train mass about 200 t) on a level stretch between Hamburg and Berlin. The fact that only four cars were used in this test run instead of the five cars normally attached to the train happened by chance due to a hot-box on a car the day before. The record run thus took place without any special preparation and almost by chance.
(Diz iz verry good tränntzläyshn! Äm DeepL(y)impressed with DeepL!)
At least it didn't have the wheel slip that the 4-4-4-4 PRR train had or that poppet valve problem.
Size was also very different, like comparing a 737 to a 747.
Beautiful that the Mallard was, the U.S. trains had something to them and the expanse of the U.S. gave opportunities that were not possible in the UK for some truly fast trains. The mountain ranges of the US are vast compared to the mere hills of the UK, if you wanted a downhill run for top speed then the U.S. should have had options. But planes came along.