NZR A class (1906)
NZR A class 428 at Glenmark Station in Waipara, 2016.
Type and origin Power type Steam Builder Addington Workshops (8), A & G Price (50) Build date 1906–1914 Total produced 58
Specifications Configuration: ​  • Whyte 4-6-2 Gauge 1,067 mm (3 ft 6 in) Driver dia. 54 in (1.372 m) Length 57 ft 2 in (17.42 m) Adhesive weight 33 long tons 2 cwt (74,100 lb or 33.6 t) Loco weight 51 long tons 0 cwt (114,200 lb or 51.8 t) 55 long tons 0 cwt (123,200 lb or 55.9 t)(simple) Tender weight 25 long tons 10 cwt (57,100 lb or 25.9 t) Fuel type Coal Fuel capacity 4 long tons 0 cwt (9,000 lb or 4.1 t) Water cap. 1,700 imp gal (7,700 L; 2,000 US gal) Firebox: ​  • Grate area 30 sq ft (2.8 m2) Boiler pressure 225 psi (1,551 kPa) (as coumpound) 190 psi (1,310 kPa)(simple) Heating surface 1,724 sq ft (160.2 m2) Cylinders 2 HP, 2 LP Cylinder size 18 in × 22 in (457 mm × 559 mm)(simple) High-pressure cylinder 12 in × 22 in (305 mm × 559 mm) Low-pressure cylinder 19 in × 22 in (483 mm × 559 mm)
Performance figures Tractive effort 17,000 lbf (76 kN) (as compound) 20,060 lbf (89.2 kN)(simple)
Career Operators NZGR Withdrawn 1954–1969 Preserved Two: 423, 428 Disposition Two preserved, remainder scrapped

The NZR A class were a class of steam locomotives built in 1906 with a 4-6-2 wheel arrangement for the New Zealand Railways Department (NZR). The class should not be confused with the older and more obscure A class of 1873. They were designed by the NZR's Chief Mechanical Engineer, A. L. Beattie and his Chief Draughtsman, G. A. Pearson to replace less powerful locomotives struggling with increasing loads on the South Island Main Trunk Railway, and in anticipation of the traffic volumes that would be created upon the completion of the North Island Main Trunk railway.^[1]

The Baldwin Q had established the Pacific as the way forward for Express passenger locomotives,^[1] but the C.M.E decided that greater efficiency was needed. The new locomotives were therefore designed as compounds. The Vauclain system had proved ineffective in New Zealand so the type attributed to Frenchman Alfred de Glehn was adopted.

The first four had Stephenson valve gear inside and Walschaerts valve gear outside, while the following 53 had just Walschaerts. The first eight locomotives were built at New Zealand Railways Department's Addington Workshops, the rest by A & G Price of Thames.^[1] The first twenty-seven were built with intercepting valves allowing full simple operation. This feature was later removed from all but the first four, and the last thirty were built without and classified as A^D until 1915.

Initially, fifty Price built engines were allocated to the North Island the rest to the South. The class were delivered with 1700 gallon tenders which were inadequate for work on the NIMT. To solve this, larger tenders were ordered for the B^B class which were given A class tenders. Delivered with saturated boilers, one of the class was given a superheated boiler after two years. Cost meant the rest were converted only as their boilers wore out. From the thirties onward the class were fitted with pressed-steel smokebox doors for Waikato spark arrestors, although not always the arrestors themselves.

Although competent engines the maintenance of the inner cylinders was difficult. In 1941 No. 582 was converted to a two-cylinder simple arrangement. The last was finished in 1949 and the class remained successful performers thereafter.^[2]

Until the arrival of large numbers of the A^B class, the A class were New Zealand's premier express engines. Starting in 1932 thirty-eight of the North Island engines went south. By this time they were being relegated to secondary and branch line service. Despite this, the last North Island engine was not withdrawn until 1961 and the last in the south until 1969, near the end of steam.^[3] The final A class to be withdrawn was A 428.

Two class members have been preserved:

• A 423 was saved by Kevin and Paul Jowett in 1970. In September 1970, the 'A' and B^A 552 were used on a steam delivery trip between Palmerston North and Frankton, with the 'B^A' leading for much of the journey. The 'A' was later put on static display at the defunct Waikato Railway Museum in Te Awamutu, which was the New Zealand Railway and Locomotive Societys Waikato Branch. Between 21 and 22 April 2008, it was moved to the Glenbrook Vintage Railway and is now in storage.
• A 428 was saved by the A 428 Preservation Society in 1973 after being stored at the roundhouse at Elmer Lane, Greymouth. It was towed from Greymouth to Christchurch by a single DJ class locomotive. Then it was towed from Christchurch to the Weka Pass Railway in Waipara with the Weka Pass Railway's recently purchased carriages. The train was hauled by another member of the DJ class and the Weka Pass Railway's two DG class locomotives on 10 December 1983. Restoration commenced in late 1988, and A 428 was recommissioned on 25 September 1993.

The eighth locomotive built, A 409, was built in 1908 as a two-cylinder simple-expansion locomotive for comparative purposes against the four-cylinder compounds.^[3] Fitted with an ALCO superheater as a trial when built, it was marginally more powerful than the other A class locomotives as a result despite having only two cylinders. Despite its differences, it was classified for a time as A^B 409 up until the 1930s along with the mechanically derived A^B class locomotives.

A 409 was withdrawn in October 1959 and was scrapped at Linwood locomotive depot as being largely non-standard.^[3]

• NZR Q class (1901)
• NZR A^A class
• NZR A^B class
• NZR G class (1928)
• Locomotives of New Zealand

• Information on the preserved A class locomotives from the Weka Pass Railway: A 423 and A 428
• A new engine at the Addington Workshops, 1906 (photo)
• NZR Steam - A/A^D class 4-6-2