The GWR class 2021 was a 0-6-0 tank locomotive introduced in 1897. It was directly based on the class 850 of 1874, but had a longer wheelbase to fit a larger firebox. The design is generally being attributed to George Armstrong, but since he retired in 1897, the innovation to build the new class might also have come from this successor William Dean. Like the class 850, the class 2021 originally also had a saddle tank.

When Churchward wanted to fit a Belpaire firebox, due to its square profile, pannier tanks had to be fitted. Most of the class was rebuilt in this way, but this process took until 1948 to be completed. By this time, British Railways took over 110 locomotives. Starting in 1949, the first ones were replaced by the class 1600 that was still based on the same design of 1874 and had dimensions which were very similar to the class 2021. In 1959, the last was withdrawn.