Wikipedia write up states: "On 11 October 1972 during a trial run it attained a top speed of 182.4 km/h or 113.3 mph. As a result, it is the fastest, operational steam locomotive in the world."

Elsewhere Wikipedia states: "Mallard is the holder of the world speed record for steam locomotives at 126 mph (203 km/h). The record was achieved on 3 July 1938 on the slight downward grade of Stoke Bank south of Grantham on the East Coast Main Line, and the highest speed was recorded at milepost 90¼, between Little Bytham and Essendine. It broke the 1936 German (DRG Class 05) 002's record of 124.5 mph (200.4 km/h)."

I guess if you search hard enough there will be various claims to be the fastes steam loco. Mallard is in the excellent York railway museum for any steam buffs visiting the UK.

Regards, Ed