The record run of the "Mallard" on July 3rd, 1938 was made with a six car streamline set plus a dynamometer car, with a total tare of 240 tons.
The Mallard pulled the train over Stoke Summit at 75 mph, then accelerated downgrade at a gradient of 1:178 to 1:200 over six miles distance to attain a speed of 114 mph. It eventually reached a speed of 125 mph, with a peak at 126 mph for a few seconds. After that it ran at a speed of at least 120 mph for another three miles.After that it ran at a speed of at least 120 mph for another three miles. By then the inside big end showed sign of overheating and the engine had to run light back to Doncaster for repair.
Can you imagine the rattling and shaking of that thing doing 126 mph, Fred Dibnah's documentary about it last night had audio of the driver:

"They knew if they smelled that aroma, they only had a few minutes until the whole big end would collapse. So at 124 miles per hour Joe Duddington smelled this aroma of violets, and instead of shutting down that engine like any normal common sense person would do. He actually opened the regulator just a little bit more, and said, "Come on girl, you can do it."

Duddington: "Go on old girl, I thought, 'we can do better than this' so I nursed her, and in the next 1 1/2 miles the needle crept up further--123 an hour, 124, 125 and for a quarter of a mile, while they tell me the folks in the car held their breaths--126 miles per hour; 126, that was the fastest steam locomotive that had ever been driven in the world."