The JA class had relatively short service lives of between 12 and 24 years as they were replaced by diesel locomotives as part of the dieselisation process of the NZR.

Unlike the J class, which were built overseas by the North British (NBL), the new locomotives were instead constructed by the NZR's Hillside Workshops in Dunedin.

During their brief prime on the South Island Main Trunk, they extended their duties from Bluff to Parnassus in the north, and as far west as Arthur's Pass on the Midland Line.

Before the full delivery of the JA class, English Electric diesel locomotives had already taken over the main freight services and summer express on the Dunedin-Oamaru section.

[3] They were particularly efficient on the relatively flat Invercargill-Dunedin section, as well as the fast run across the Canterbury Plains between Christchurch and Oamaru.

[1] According to anecdotal accounts around the performance of the JA class on the South Island Limited during the late 1960s, the speedometer was reading over 75 miles per hour (121 km/h), and the train was moving away rapidly from the traffic on State Highway 1 adjacent to the railway.

[6] Rail writer David Leitch believed that the drivers' accounts of 85 miles per hour (137 km/h) running were accurate.

[7] Additional support comes from an official source, the 1966 Encyclopaedia of New Zealand, which states that "The speedy and graceful "JA" has demonstrated its remarkable performance in express service on the level plains of Canterbury, and on occasions has attained speeds in excess of 70 miles an hour.

"[8] The official NZR speed record of 125.5 kilometres per hour (78.0 mph) was set by a Vulcan RM class railcar on a delivery trial run in 1940.

[9] The use of steam locomotives on freight trains on the East Coast of the South Island ended in March 1969.

It was also planned to dieselise the overnight Friday and Sunday express with surplus steam heat vans when the North Island Limited was replaced by the Silver Star, but delayed arrival of the carriages from Japan by a year meant that steam heat vans, which allowed diesel to operate night trains, would not be available until late 1971.

It was evident in 1950 that additional motive power was required in the North Island, but dieselisation had not yet begun.

In April of the same year, the NZR Chief Mechanical Engineer requested the order be changed to oil-burning due to the perceived long-term coal shortage caused by the 1951 New Zealand waterfront dispute.

Other detail differences were 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 absence of a smokebox number plate (although a smokebox plate was specified by the NZR).

Described by the author Stewart as "fine clean-lined machines",[14] they handled almost every express train in that region for about twelve years.

By comparison, the Palmerston North-based JA class spent more time working freight trains, particularly with the arrival of the 88-seater railcars.

The rest of the steam fleet, excluding the J and JB classes, experienced increased mechanical failures and repair costs while moving the heavier post-war traffic on the NIMT.

The cost per mile for the JA was much higher, with the locomotives achieving 82% of the DA class' mileage.

Although JA 1286 was briefly transferred to Auckland in the mid-1960s, it was felt that it did not perform as well as the eleven Auckland-based locomotives and was quickly reallocated to Frankton.

They were rebuilt to accommodate a coal bunker in place of the fuel oil tank and coupled to J-class locomotives whose original 1939-built tenders were life-expired.