A minor visual difference between the 5400 and earlier 6400, and the later series of 6400, with the 7400 classes was at the join between cab and bunker.

Thus the basic design was almost sixty years old when new, the 4 ft 7+1⁄2 in (1.410 m) driving wheels being the main distinguishing factor, apart from the more modern profile.

There were also superficial similarities with the GWR 645 Class as extant in the 1930s, that also had 4 ft 7+1⁄2 in (1.410 m) wheels and 24 in (610 mm) stroke cylinders (and by then pannier tanks and full cabs).

The smaller wheels of the 6400's permitted operation in hillier locations than the 5400 Class and allocations were initially to the South Wales valleys.

[4] Being allocated to Plymouth Laira the type was trialled on the former Lostwithiel and Fowey Railway, although a 1400 Class 0-4-2 engine was normally used.