by the NZR). In service Captive to the North Island, the North British JAs operated on the routes most commonly worked by oil-burning locomotives. "The North British JAs 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. 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 class were rebuilt with KA frames in 1955–57 as it unacceptable to write off the K class locomotives, and retain useful Baldwin Aa. However, the superiority of the DA class diesel and JA meant it was announced, an "A grade" overhaul of a K class locomotive in 1961 was the last for the class. An analysis in 1959 found that the North British JA matched the availability of the new DA class, each JA being available 252 days a year in the North Island. Cost per mile for the JA was much higher, with the steam locomotive achieving 82% of the DA mileage. The first eleven, JAs 1275–1285, were based in Auckland while the other five, JAs 1286–1290, were based initially at Palmerston North before being moved to Napier in 1963. The Auckland-based locomotives were regularly allocated to the North Auckland Line the Opua Express until 1959 and the Helensville local until 1966 and the North Island Main Trunk services such as Auckland–Wellington, 227/626 Express Mail train 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