The 710 had been fitted with a Franklin C-2 booster engine[1] on the trailing truck and this was repeated for the 720.

[2] Partly to reduce this axle load, partly to support the increased weight of the enlarged firebox, the 720 changed from the 710's 2-8-2 Mikado wheel arrangement to a 2-8-4 Berkshire.

The trailing wheels of this bogie were of different sizes, the larger rear wheels being driven by the booster and a smaller wheelset used under the front of the ashpan to give more clearance.

The final twelve differed in having streamlined valances and boiler lagging tinware.

Coal shortages after World War II saw 14 converted to burn oil.