The NZR Ja Class Steam Locomotives were the later version of the J Class.
Fuel - Coal / North British Ja's Oil
Weight - 109.45 Tons
Wheel Arrangment - 4-8-2
Running Numbers - 1240 to 1290
1 at Steam Incorporated Paekakariki
3 at Mainline Steam Auckland - Wellington - Christchurch
1 at the Glenbrook Vintage Railway Auckland
1 at the Plains Railway
1 at the Otago Settlers Museum
The class were an improved version of the J class locomotives. Unlike the J class, which were built overseas by the North British Locomotive Company in Glasgow, Scotland, the new class were instead constructed by NZR's Hillside Workshops in Dunedin. However, much of the components, such as the bar frames, were still sourced from North British  as Hillside lacked the means to construct such a frame.
Some of the more notable improvements from the J class were the use of the Westinghouse cross-compound pump; Ashton double pressure gauge; the Sellars injector in the cab; steam for the pump was taken from a manifold in the cab and not an outlet on the steam dome; combined gauges in the cab; and roller bearings on the driving and connecting rods. However, the first two JAs produced, No.'s 1242 and 1243, did not have roller bearings on any of the rods, and on the rest of the locomotives numbered in the 124X range, roller bearings were only present at the connection between the driving and connecting rods and not on the other wheels. The "all rolling bearing JA... 1265 to 1270" were assigned by Linwood locomotive depot to the South Island Limited wherever possible  between 1952 and 1968, the fastest being 'legendary' JA 1270, scrapped in mid 1969, requiring only minimal repair after running into an unaligned turntable pit. In most other respects however, the class was virtually identical to the earlier J class, although the JA class was never fitted with bullet-nose streamlining.
Although the first class member was built in 1946, the boilers for the last 10 Hillside JA plus two spare boilers were not delivered from North British until 1953. For this and a number of other factors, the last member of the class was not turned out of the workshops until December 1956. That last locomotive, JA 1274, was both the last steam locomotive built for, and by, NZR. The class was intended for the far more extensive fast passenger trains that existed in the South Island before the introduction of Fiat railcars in 1956 and the transfer of passengers to air services.
The class only ever worked in the South Island during their NZR career. The first members operated out of Dunedin, although they were supposed to be allocated to Christchurch. When enough of the class were in service to displace the 4 members of the J class based out of Dunedin, those locomotives headed north to join the rest of their class in the North Island. The class performed their duties over most South Island lines, as far south as Bluff, although axle loading and gauge profile restricted there use on the North line to Picton, although some of their fastest running, over 60mph was achieved on the night Cabbage Train, 111/112 in 1957-63  with 1-3 carriages and express goods, on the Christchurch Kaikoura sector  The JA was too big for long branch lines like Otago Central Railway or Kingston Branch. Despite this, the class became the mainstay of the South Island network. It excelled on the South Island Limited on the fairly flat, Invercargill-Dunedin section and particularly on the fast run across the Canterbury Plains between Christchurch and Oamaru, and the evening run back on 144 to Christchurch was legendary when late and racing to get to the Lyttelton-Wellington Ferry. Reliable footplate observers of the JA on the South Island limited during the late 1960s saw the speedometer over 75 miles per hour (121 km/h) and the train moving away rapidly from the traffic on State Highway One adjacent to the railway. Rail writer David Leitch, a Masterton Lawyer believed JA drivers account of 85 miles per hour (137 km/h) running as accurate. The maximum official speed was 100 kilometres per hour (62 mph), but the class operated effortlessly at high speed, especially across the Canterbury Plains. JA 1267 pulling ten carriages weighing 300 tons covered the 42 miles (68 km) from Tinwald to Washdyke at 59.4 miles per hour (95.6 km/h). The official NZR speed record of 125.5 kilometres per hour (78.0 mph) was set by an Vulcan RM class railcar, but the JA class unofficially operated at higher speeds. Another duty for the class was the Bluff school train, which ran from Bluff to Invercargill and back every school day until its replacement by buses in the mid-1960s. There were (and still are) no secondary schools in Bluff.
The first of the class, JA 1249 and 1272 were not withdrawn until March 1968, in distinct contrast to the types J, JB and North British JA cousins. In 1966, 12 JA class members (and four J class) received a full overhaul, six in 1967 and two in 1968.  Use of steam on freight on the East Coast of the South Island finished in March 1969, and 15 JAs and the three Js rebuilt with North Island JA tenders and trailing engine bogies were maintained for service. JA 1267 was the last steam engine given an A grade overhaul by NZR in November 1968, and with the last three Js provided one of the last high speed steam services in the world, between Christchurch and Oamaru. The JA class lasted in front-line duty until the end of steam in New Zealand; the South Island Limited was then worked by steam locomotives until replaced by the diesel Southerner in November 1970. It was also planned to dieselise the overnight Friday Sunday express with surplus steam heat vans when the North Island Limited was replaced by the Silver Star, but late delivery of the new train from Japan meant the services were steam hauled for another year until 26 October 1971. The locomotives remained ready for service into November but were never called upon.
Due to locomotives suffering mechanical defects or in need of repairs, they were often sidelined. Some, such as JA 1271, were used as stationary boilers. These locomotives, as was common practice at the time, were often stripped of parts to keep the rest of the fleet going.
North British Built Ja's
In 1950 it became clear additional motive power was required in the North Island, but the process of dieselisation was yet to begin. Consequently, NZR chose to order 16 steam locomotives from North British to the design of the successful J class. These locomotives contained a number of differences to both the J class and Hillside JAs – although turned out with the cross-compound pump, roller bearings on the rods were limited to the connection between the connecting and driving rod, mechanical lubrication was employed. In January 1951 the order had been made for the 16 JA to be more coal burners, and North British regarded the order as essentially a repeat order, virtually identical to 1939 J, but in April 1951 the NZR CME requested the order to be changed to oil burning due to perceived long term coal shortage due to the waterfront strike and the associated strike by miners and unavailability of shipping for coal and the expected long term high imported coal price and the long time it would take to build up coal stocks to safe levels, and the class were built as oil burners, with no grate, ash pan or fire door (the only class of locomotives on the NZR to be built completely as oil burners)and ACFL blow down was incorporated late in their construction. A significant improvement was the incorporation of French T.I.A , blow down equipment which enabled rapid ejection of boiler sludge  and reduced the boiler scale, enabling much faster turnaround and higher availability. The system of injection of the oil flow into the burners is different and far more effective than in NZR 1948-50 conversion of 12 Js to oil burning. Other detail differences was the use of Stone's headlights and electrical generator instead of the usual Pyle National equipment, the "Butterfly" number boards on the front headlight, and the lack of smokebox number plate (although a smokebox plate was specified by the NZR).
Captive to the North Island, the North British JAs operated on the routes most commonly worked by oil burning locomotives. "The North British JA's were fine clean lined machines, and extremely popular in the Auckland district where they were put to use on most tasks. They handled almost every express train in that area for a dozen years,"  The Auckland JA were very much specialized express engines and far more impressive express performers than the KA in the view of most Auckland engine drivers. The North Island JAs in their early years were the most efficient rail engines in New Zealand, and outclassed the early English Electric diesel locomotives and K class. However the rest of the steam fleet, other than the J and JB, suffered increased mechanical failure and repair costs, in moving the heavier post war traffic on the NIMT. By 1955 the K and X 4-8-2 classes were worn out, but 9 K were rebuilt with KA frames in 1955-57 as it unacceptable to write off the legendary K and retain useful Baldwin Aa. However the superiority of the DA and JA meant it was announced, a A overhaul of a K in 1961, was the last for the class, but sensitivity delayed bloc withdrawal till 1964. During the 1955-59 period the North Island JA matched the availability of the new DA class, each JA being available 252 days a year in the North Island, and achieving 82% of the DA mileage but the steam engines required biannually overhauls and fuel costs were 3 times that of the DA. The first eleven, JAs 1275–1285, were based in Auckland while the other five, JAs 1286–1290, dubbed the 'Forgotten Five' by enthusiasts – were based initially at Palmerston North before being moved to Napier in 1963. Had the North Island JA oil fired ever run in the South Island, it quite possible that they would have been capable of 100 miles per hour (160 km/h), as their efficient acceleration is unmatched among the J variants and the J rail classification is that of the fastest passenger engines.
The Auckland-based locomotives were regularly allocated to the North Auckland Line the Opua express till 1959 and the Helensville local till 1966 and the North Island Main Trunk services such as Auckland–Wellington, 227/626 Express (Mail) and faster Night Limited and Auckland–Wellington express goods Train 627. There are however accounts of them being used elsewhere, with accounts surviving of JA 1279 running on the Waiuku Branch in the early 1960s, and later on the Raetihi Branch during the same decade after coal-fired AB 700 caused several lineside fires. When working with other 4-8-2s on the NAL, they were required to have a bogie wagon such as a UB flat wagon between the two engines in order to distribute weight more evenly on the light bridges along that line.
The five Palmerston North-based engines by contrast were less frequently allocated to NIMT trains, instead running over the Marton - New Plymouth Line to Wanganui and the Palmerston North - Gisborne Line as far as Gisborne. They were considered to be harder working than their Auckland counterparts, which spent most of their time rostered to passenger services. By comparison, the Palmerston North-based JAs spent more time working freight trains, particularly with the arrival of the Drewry 'Twin-set' railcars.
Following the onset of dieselisation in the 1960s, the locomotives migrated to Frankton Junction. While JA 1286 was briefly transferred to Auckland in the mid-1960s, it was felt it did not perform as well as the eleven Auckland-based locomotives and was quickly re-allocated to Frankton. After this, the locomotives were largely relegated to the old East Coast Main Trunk Railway between Hamilton, Tauranga and Taneatua, again largely on freight trains due to the reduction of passenger services from Taneatua to Te Puke, which were now handled by the 'Twin-sets'.
Despite being a very young class, some of the North British JA class members were among the first of the J types to be withdrawn. The first of the class to be withdrawn, JA 1279, was withdrawn in 1964 and sent to Hillside Workshops minus at least one driving wheelset, taken to repair JA 1275 after it suffered an axle fracture while passing through Mercer that year. The rest of the locomotive then became a source of spares for the J and Hillside JA class locomotives in the South Island, with the oil-fired boiler being converted to coal-firing before being fitted to Greymouth-based J 1212 during a C-grade overhaul.
As the North British JAs were withdrawn, most were stripped of parts to keep the South Island J and JA class locomotives running, along with the remaining North British JAs which by then were based out of Frankton Junction. Several of the tenders from JA 1287, 1288, 1290 locomotives, transferred south in November 1966, which were in relatively good condition, were rebuilt to accommodate a coal bunker in place of the fuel oil tank and attached to J class locomotives whose original 1939-built tenders were life-expired. Other parts of the surplus North Island JA were also refited,including the trailing trucks and approval was given to fit JA 1288 boiler. Most of the North Island JA were withdrawn too late in 1967 or early 1968, to be reprocessed, as A grade SI J overhauls had ceased with diesalisation.
Only three of the North British JA class managed to reach the end of North Island steam in 1968, JA 1275 being one of them. After withdrawal and removal of all useful parts, the North British JAs were onsold to Sims Pacific Metal Industries and towed as required to Sims-PMI's Otahuhu scrapyard, adjacent to the Otahuhu Workshops, for scrapping. The only locomotive to avoid this fate, JA 1275, was purchased by Les Hostick in 1968 and transported to the NZR&LS Waikato Branch's Te Awamutu Railway Museum along with BB 144 for static display.