title: LNER Class A4 4468 Mallard
context: London and North Eastern Railway locomotive numbered 4468 Mallard is a  Class A4 4-6-2 Pacific steam locomotive built at Doncaster Works, England in 1938. It is historically significant as the holder of the world speed record for steam locomotives at 126mph (203km/h).[1]
question: What was the fastest steam locomotive?
answers: Mallard