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.
4-6-4
The Chicago, Milwaukee, St. Paul and Pacific Railroad (Milwaukee Road) could have produced the first American since its design work was done earlier than that of the NYC, but financial constraints delayed the project and the Milwaukee's locomotives only emerged in 1930. The Milwaukee called them Baltic, following the European practice started in France. The initial order of fourteen Class F6 locomotives was followed by eight more Class F6a locomotives in 1931 and, in 1938, the Milwaukee acquired six streamlined Class F7 Hudsons with the shrouds. These took over the Milwaukee's crack "Hiawatha" express trains from the Class A Atlantics and were among the fastest steam locomotives of all time. The schedules of many of these trains required extended running at .