 These tank engines were based on the previous Aspinall Class 30 0-8-0 tender engines, although their similarities have often been over-emphasised.Their coupled wheelbase was extended by two feet to , requiring the two centre drivers to be flangeless, with widened tyre treads, to allow them to negotiate a tight radius curve within a marshalling yard.This was more successful than similar flangeless drivers had been with Hoy's Class 26 2-6-2Ts, where they had shown a tendency for the centre drivers to drop between the rails if track in a siding wasn't maintained as well as main-line track.