On the 3rd July 1938, on Stoke Bank, near Grantham in Lincolnshire, UK, the London and North Eastern Railway (LNER) A4 Pacific (4-6-2) locomotive "Mallard" set the world record for a steam powered train at 126 MPH (203 Kph). The train Included a dynamometer car and was ostensibly conducting brake testing. The record has yet to be broken and, with no current contenders, is likely to remain.

"Flying Scotsman", an earlier LNER A3 Pacific, was the first to be officially recorded at 100 MPH (160 Kph), and “Princess Elizabeth” was a contemporary Pacific locomotive running on the rival London and North Western Railway (LNWR). "Tornado" is a recent Pacific locomotive, built by private enterprise.

More Info: en.m.wikipedia.org