Although designed to work on the lighter secondary lines, the class was frequently used on mainline express passenger trains as well as freight.

The order, initially to be for 30 locomotives, was placed overseas with Britain despite criticism, because of the urgency of the requirement and the limited capacity of the NZR Workshops.

[2] They were immediately put into service on the main trunk routes in both islands to help transport wartime traffic during World War II.

The conversion process involved installing a two-nozzle burner in the firebox, removing the grate and ashpan, and replacing them with a firepan lined with bricks.

The conversion also required the addition of related controls and gauges for the oil-burning equipment, and modifications to their Vanderbilt tenders to carry an oil bunker and associated steam piping.

Similar to the K and KA classes that were also converted to oil burners at that time, these tenders utilised a separate, removable tank that sat in the former coal space.

Several locomotives of the JB class were among the first J 4-8-2 types to be withdrawn due to faster wear and tear resulting from oil burning.

[7] The last three Js to receive A-grade overhauls in 1967 were 1211 (reboilered with a spare North British JA boiler supplied in 1953, which had been used on JB 1230 from 1959 until it was scrapped in 1964), 1234, and 1236.

The delayed arrival of the North Island Silver Star 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.

[8] Half a dozen other JA class remained usable but in dubious condition for use on the weekend 189/190 trains until the end of steam on 26 October 1971.