The Baldwin Q had established the Pacific as the way forward for Express passenger locomotives, 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. 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 AD until 1915.
Service and modifications
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 BB 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. Until the arrival of large numbers of the AB 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 Despite this, the last North Island engine was not withdrawn until 1961 and the last in the south until 1969, near the end of steam. The final A class to be withdrawn was A 428.
Preservation
Two class members have been preserved:
A 423 was saved by Kevin and Paul Jowett in 1970. In September 1970, the 'A' and BA 552 were used on a steam delivery trip between Palmerston North and Frankton, with the 'BA' 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.
A 409
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. 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 AB 409 up until the 1930s along with the mechanically derived AB class locomotives. A 409 was withdrawn in October 1959 and was scrapped at Linwood locomotive depot as being largely non-standard.
