The Patriot Class was a class of 52 express passenger steam locomotives built for the London Midland and Scottish Railway. The first locomotive of the class was built in 1930 and the last in 1934. The class was based on the chassis of the Royal Scot combined with the boiler from Large Claughtons earning them the nickname Baby Scots. A total of 18 were rebuilt to create the LMS Rebuilt Patriot Class between 1946 and 1948; thereafter those not subjected to rebuilding were often referred to as the Un rebuilt Patriot Class. These remaining 34 Un rebuilt engines were withdrawn between 1960 and 1962.

All of the original un-rebuilt LMS Patriot Class locomotives were withdrawn by November 1962 and the whole class was withdrawn by December 1965 with none of the engines (un-rebuilt or rebuilt) surviving into preservation; however, a replica of no.5551 is under construction. The LMS-Patriot Project, a registered charity, is building a replica which will carry the number of the last build: LMS number 5551.

No.5551 The Unknown Warrior is a "rebuilt" or replica LMS Patriot Class steam locomotive which is under construction. The locomotive is a "rebuilt" of the final 'Patriot' class locomotive to be built, No.5551. The original locomotive was built in May 1934 and withdrawn in June 1962 (it was scrapped that October). Unlike the original engine, which never received a name, the replica will be named "The Unknown Warrior"; the name was chosen by public poll.