NZR Ab class

NZR A^B class

Preserved AB 778 hauling the Kingston Flyer

Type and origin Power type Steam Builder NZGR Addington Workshops (38) North British Locomotive Company (83) A & G Price Limited, Thames (20) Build date 1915–1918, 1921–1927 Total produced 141

Specifications Configuration 4-6-2 Gauge 3 ft 6 in (1,067 mm) Driver dia. 54 in (1.372 m) Wheelbase 52 ft 6 in (16.00 m) Length 62 ft 5 in (19.02 m) Loco weight 51.3 long tons (52.1 tonnes; 57.5 short tons) Tender weight 33.4 long tons (33.9 tonnes; 37.4 short tons) Total weight 84.7 long tons (86.1 tonnes; 94.9 short tons) Fuel type Coal Fuel capacity 4.0 long tons (4.1 tonnes; 4.5 short tons) Water cap 3,500 imp gal (16,000 l; 4,200 US gal) Firebox:  • Firegrate area 33 sq ft (3.1 m2) Boiler pressure 180 lbf/in2 (1.24 MPa) Heating surface 1,148 sq ft (106.7 m2) Superheater: ​  • Heating area 204 sq ft (19.0 m2) Cylinders Two, outside Cylinder size 17 in × 26 in (432 mm × 660 mm)

Performance figures Maximum speed 60 mph (97 km/h) Tractive effort 20,030 lbf (89.10 kN)

Career Operators NZR Numbers 608–838 Locale All of New Zealand First run October 1915 Withdrawn 1963–1969 Preserved 7 (608, 663, 699, 745, 778, 795, 832) Current owner Kingston Flyer, Mainline Steam, Pleasant Point Museum and Railway, Museum of Transport and Technology, Ferrymead Railway, Rimutaka Incline Railway Heritage Trust. Disposition Seven preserved, two lost in the wreck of Great Barrier Island in 1922, remainder scrapped.

The NZR A^B class was a class of 4-6-2 Pacific tender steam locomotive that operated on New Zealand's national railway system. Originally an improvement on the 1906 A class, 141 were built between 1915 and 1927 by NZR's Addington Workshops, A & G Price Limited of Thames, New Zealand, and North British Locomotive Company, making the A^B class the largest class of steam locomotives ever to run in New Zealand. An additional ten were rebuilt from the tank version of the A^B – the W^AB class – between 1947 and 1957. Two North British-made locomotives were lost in the wreck of the SS Wiltshire in May 1922.

The genesis of the A^B class originated from the construction of A class 4-6-2 N^O 409 at Addington Railway Workshops in 1906. A two-cylinder simple-expansion locomotive, 409 was initially classified A^B to differentiate it from the four-cylinder compound A and A^D class locomotives, which were by and large of a similar design although built as compounds. A^B 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, A^B 608, was to a completely different design from A^B 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 A^B 409. It also had the distinctive Vanderbilt tender, which would become a hallmark of the A^B 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.

The A^B 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 (16.4 W/kg), the A^B 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 A^B 663. New tender structures were also built at a later date.