NZR AB Class - Introduction

Introduction

The AB class compared more favourably in service against the compound A class. Reputedly the first engine able to generate one horsepower for every 100 pounds of weight, the AB class was efficient and versatile, and the engines were easy to maintain and operate. However, it became clear that there were several shortcomings of the design - notably the cab was too small, and the tenders were not sturdy enough. A new and longer cab was fitted to all new locomotives being built from that time onwards, with the first appearing on AB 663. New tender structures were also built at a later date.

The locomotives were extremely versatile, and the AB class were used on almost every section of the NZR network, the exception being the Nelson Section (closed 1955). In later years, the class were displaced from their mainline duties first by the larger J and K series locomotives, and later by the arrival of the DA, DG/DH, and DJ class diesels in the North Island (DA and DG classes) and the South Island (DH and DJ). As a result of the arrival of new diesels in the North Island from 1955 onwards, many of the North Island based locomotives were transferred to the South Island to see out their remaining working lives.

The final duties of the AB class were on branchline workings, where they found their niche after being displaced from most mainline duties by larger locomotives. These in turn displaced the A and Q class 4-6-2 locomotives from their duties, resulting in the withdrawal of all Q class locomotives by 1957 and a reduction in the number of A class locomotives. 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.

Being highly capable, the AB class were used for both freight and passenger trains. The AB class was easily able to pull an express passenger train at speeds of 100 km/h (62 mph), or haul 700-tonne goods trains on easy grades. They, along with the earlier 'F' class, were known as the 'Maids of all work'. Their work on express passenger trains was diminished by the arrival of the J class of 1939, and K class of 1932 tender locomotives in the 1930s, and also with the introduction of the heavier steel-panelled carriages built from 1930 until 1943. Despite being displaced, the AB class could still be found at work on relief expresses during holiday periods.

The locomotives did not change greatly during their NZR career. The first change was to fit Waikato-type spark arrestors to many of the North Island based engines, resulting in a new 'pear-shaped' smokebox with an ash hopper at the base of the smokebox. The cast-iron smokebox doors were replaced by steel ones, and the brake pump was moved from its original location on the right-hand side of the smokebox to a new position, recessed into the running board, just forward of the cab on the same side.

Another notable change was that many of the class were fitted with ballast blocks for mechanical purposes. As the locomotives rode well, this was not to prevent derailments as with the Q class 4-6-2s of 1901. Furthermore, certain engines were not fitted with ballast blocks, so their inclusion is questionable. 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.