NZR AA class

From Wikipedia, the free encyclopedia
Jump to: navigation, search
NZR AA class
Type and origin
Power type Steam
Builder Baldwin Locomotive Works
Build date 1915
Total produced 10
Configuration 4-6-2
Gauge 3 ft 6 in (1,067 mm)
Driver dia. 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)
Total weight 88.5 long tons (89.9 tonnes; 99.1 short tons)
Fuel type Coal
 • Firegrate area
28.3 sq ft (2.63 m2)
Boiler pressure 170 psi (1.17 MPa) (as coumpound)
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)
Operators NZGR
Numbers 648 - 647
Withdrawn 1955–1957
Disposition All scrapped

The New Zealand AA 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[edit]

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 QB 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 War 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.[1]


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 AB class. Not an issue with hard coal, increasing use of soft Waikato coal meant a larger grate was desirable.[2] Thus entire class had their boilers replaced with AB 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.In the mid 1950s most of the class were still in reasonable condition and the engines were regarded as a very successful class, capable of taking somewhat heavier loads than an Ab. Serious consideration was given to a major refit of the class rather than rebuilding 9 of the mid 1930s K class which were essentially worn out, and very expensive to run and repair, after being run hard and fast to cope with heavier freight traffic in the early 1950s and the attempts of enhusiastic drivers to outrun the new diesel competition. The option of modernising the Aa class was rejected on account of their age, and due to the fact the cultural importance of the monumental K's made them impossible to withdraw in 1956. The first withdrawals occurred in December 1955, when six were taken out of service with their boilers returned to the AB class boiler repair pool. The remaining four continued to operate until February 1957. None are preserved.

Class Roster[edit]

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



  1. ^ Millar, Sean (2011). The NZR Steam Locomotive. NZR&LS. 
  2. ^ Millar, Sean (2011). The NZR Steam Locomotive. NZR&LS. 
  3. ^ Register of New Zealand Railways Steam Locomotives 1863-1971 ISBN 0-9582072-1-6

External links[edit]