Mainline steam trains in Great Britain
The most famous steam locomotive operating on the British main line is the 1923 built "Flying Scotsman". After being taken into public ownership in 2004, following a decade long refit it returned to mainline service in 2016. The fastest steam locomotive ever built, the 1938 built "Mallard", was also certified for main line operation briefly in the 1980s.
James Holden (locomotive engineer)
The Decapod developed mainly under Chief Draughtsman Frederick Vernon Russell was an extraordinary endeavour to develop a steam locomotive which could perform at the level of electric traction. It was built in 1902 to forestall an imminent scheme for an electrified railway out of London to suburbs served by the GER. Since the proponents of the scheme had a slogan about electric trains accelerating to thirty miles an hour in thirty seconds, Holden resolved to obtain the same performance with steam traction. A massive boiler with Wootten firebox, three cylinders each with its own blastpipe cone, and ten smallish driving wheels ensured a lively acceleration. On trial it did rather better than 30 mi/h (50 km/h) in thirty seconds, accelerating at 1.46 ft/s² (0.45 m/s²): This performance put an end to the electrification scheme even though (as Holden had known all the time) the regular use of so massive a machine would never have been permitted by the civil engineer.