NZR AB Class - Construction and Design

Construction and Design

The genesis of the AB class originated from the construction of A class 4-6-2 NO 409 at Addington Railway Workshops in 1906. A two-cylinder simple-expansion locomotive, 409 was initially classified AB to differentiate it from the four-cylinder compound A and AD class locomotives, which were by and large of a similar design although built as compounds. AB 409 was in reality the 'guinea pig' 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.

The new locomotive, AB 608, was to a completely different design from AB 409 when it emerged from the Addington Workshops 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. It also had the distinctive Vanderbilt tender, which would become a hallmark of the AB class during its working life. This locomotive was to become the first of 141 similar locomotives built by various builders in New Zealand and the United Kingdom for NZGR.