AB 409 was, in reality, the experimental locomotive for what would become the most prolific type on the New Zealand Railways network, with construction beginning on a new prototype 4-6-2 in 1915.

Although largely similar to the A class, it had a new design of cab and boiler, which was fitted with a superheater after trials conducted on AB 409.

A new and longer cab was fitted to all new locomotives being built from that time onwards, with the first appearing on AB 663.

The locomotives also worked some lesser mainline duties, and others, such as those at Gisborne, were used as bankers to assist trains heading south to Napier.

Despite being displaced, the AB class could still be found at work on relief expresses during holiday periods.

Other cosmetic changes included fitting a large Pyle National electric headlight on top of the smokebox and replacement of the copper-capped funnels with the standard NZR "flowerpot" type.

These boilers were of standard construction, no matter which firm built them, and as such were interchangeable across any locomotives of the type.

The replacement took place in the 1930s, and these locomotives gained new heights of reliability, before they were withdrawn in 1957 (at the same time as the Q class).

The tenders, although satisfactory, suffered from having been constructed too lightly for the intended task, and were also prone to rusting, particularly around the frame channels.

Shopped for an 'A' grade overhaul at Hillside Workshops, the locomotive had a cracked mainframe but was otherwise in mechanically good condition.

It was decided to use part of the good-condition frames from under AB 661, then recently withdrawn, to replace the damaged section from 792.

However, with 661 written off and 792 being a younger locomotive that still had an economic career ahead, the NZR would not have considered 'returning' 661 to service and 'scrapping' 792.

With interest for a nostalgic steam-operated train growing, both 778 and 795 were restored that same year for the famous Kingston Flyer.

AB 663 was used initially at Dunedin for spare parts to keep 778 and 795, now named David McKellar and Greenvale respectively.

[5] AB 608 gained the notable distinction of being named Passchendaele in 1918 to commemorate the NZR staff who had been killed in the First World War.

[6] The locomotive had its nameplates removed in the 1940s, and they were placed on display in the Christchurch and Dunedin railway stations.

Two replicas were made in 1963 for the NZR centenary event, and these are held by the New Zealand Railway & Locomotive Society.