The new carriages had different bogie spacing and a more rounded profile, to allow general route availability.

This primarily reflected the carriages use – second class (X25480), first-class, first and second-class composite (X25916) or semi sleeping.

56-foot carriages quickly became the mainstay of the passenger fleet, being used on the NZRs important trains in their red livery.

NZR began overhauling a number of 56-foot carriages for a new South Island train called the "Southerner".

Classified AB, these carriages had a kitchenette and a full-length buffet counter installed.

A similar set of carriages was upgraded in the North Island for the new "Endeavour" train from Wellington to Napier.

These carriages were the first to be turned out with the new "Supavent" style of windows,[1] and were painted a mustard yellow instead of blue.

Other carriages were given upgraded seating and new paint colours but remained on the older style bogies and without air conditioning, for commuter and charter use.

[11] In September 2007 Dunedin Railways announced it had purchased twelve small-window 56-foot carriages formerly used on the Wairarapa Connection.

Of note, all of the AO/ASO big window 56-foot carriage variety were sold to either Dunedin Railways or heritage organisations, although ASO14 was later scrapped by Mainline Steam due to its poor condition.

Many other groups own examples of 56-foot carriages, but only one of the original 1927 Prototype batch survives - this being Aa 1618 which is in use by Steam Incorporated as a buffet car.