Mike that engine was designed by one: Nigel Gresley. To be fair Gresley was part of a team. But it was his genius that lead to the A1 Pacifics becoming the first steam locomotive to exceed 100mph. A1 Pacifics was the name of the class of engine that Flying Scotsman belongs to. Gresley went on to design the aerodynamic A4 Pacific, of which, Mallard belongs to. Mallard set the world land speed record for a steam train on July 3rd, 1938. The train achieved 125.88mph (202.58kph) A record that still stands.