This kit represents those carriages built after the prototype vehicle (S15000) which had slightly different internal dimensions. Four first class and three third class (reclassified second in 1956) compartments were separated by a cross vestibule with a door in the compartment side. As well as two doors each end there were two extra doors on the corridor side. The central third class compartment had a top-hinged opening window to admit stretchers. D126 had six seats per compartment with retractable armrests; D128 third class compartments had eight seats. Some of the SR allocation of D128 were converted to EMU carriages for the Bournemouth electrification.