NZR AA class

NZR A^A class

Type and origin Power type Steam Builder Baldwin Locomotive Works Build date 1915 Total produced 10

Specifications Configuration 4-6-2 Gauge 3 ft 6 in (1,067 mm) Driver diameter 49 in (1.245 m) Length 34 ft 9 in (10.59 m) Adhesive weight 34 long tons 14 cwt (77,700 lb or 35.3 t) 34 long tons 14 cwt (35.3 t; 38.9 short tons) Loco & tender weight 88.5 long tons (89.9 tonnes; 99.1 short tons) Fuel type Coal Boiler pressure 170 psi (1.17 MPa) (as coumpound) Firegrate area 28.3 sq ft (2.63 m2) Heating surface 1,465 sq ft (136.1 m2) Cylinders 2 Cylinder size 18 in × 24 in (457 mm × 610 mm)

Performance figures Tractive effort 21,580 lbf (96.0 kN)

Career Operators NZGR Numbers 648 - 647 Withdrawn 1955–1957 Disposition All scrapped

The New Zealand A^A class consisted of ten steam locomotives built by Baldwin Locomotive Works in 1914 to an order by Chief Mechanical Engineer, H. H. Jackson for operation on New Zealand's national rail network.

Introduction and Design

Built to a similar but vastly improved design to the Q class of 1901, they had a wheel arrangement of 4-6-2 and were suited to hauling freight services in the North Island (Originally they were to be classed Q^B but as they were to supplement the A class, given their aforementioned designation.) The United States built locomotives were an urgent order needed due to heavy demand with both New Zealand and British workshops unable to supply as the First World World was draining resources and manpower for the war effort. Construction was completed less than two months after the order was placed and all ten entered service in New Zealand in June 1915. Bar frames were used by the American builders instead of NZR's preferred plate frames, and the class had superheaters as built. Water capacity was double that of the A class and coal a quarter more.

Service

They were worked hard for four decades being supplanted by the more powerful "K" and "J" class families. In 1919 No. 654 was used for a very short experiment in the use of pulverized coal. Only one trip is known to have been made before the engine was converted back. They were initially equipped with grates smaller than either the A or A^B class. Not an issue with hard coal, increasing use of soft Waikato coal meant a larger grate was desirable. Thus entire class had their boilers replaced with A^B class types starting in 1940 giving the reliable locomotives more life. This soon became a blessing when they were worked hard through the Second World War. They were used extensively on the steeply graded central NIMT, Wanganui and Gisborne sections. The first withdrawals occurred in December 1955, when six were taken out of service with their boilers returned to the A^B class boiler repair pool. The remaining four continued to operate until February 1957. None are preserved.

Class Roster

Key:	In service	On lease	Out of service	Preserved	Overhaul/Repair	Scrapped

Road number	Builder	Builders number	In service	Written off	Comments
648	Baldwin	41826	6-1915	22-12-1955
649	Baldwin	41827	6-1915	2-1957
650	Baldwin	41828	6-1915	2-1957
651	Baldwin	41829	6-1915	2-1957
652	Baldwin	41830	6-1915	22-12-1955
653	Baldwin	41831	6-1915	22-12-1955
654	Baldwin	41832	6-1915	22-12-1955	Experimentally converted to burn pulverised coal 1921. Converted back to hard coal 1922.
655	Baldwin	41833	6-1915	22-12-1955
656	Baldwin	41834	6-1915	2-1957
657	Baldwin	41835	6-1915	22-12-1955

External links

• Baldwin Steam Trust - New Zealand locomotives built by Baldwin, including A^A class statistics
• Plan of an A^A class locomotive by Derek Brown