The new locomotives incorporated a number of improvements, including a re-designed plate frame to eliminate the cracking issues the K class were experiencing; roller bearings on all wheels; hydrostatic lubrication throughout; and the inclusion of the ACFI feedwater heater system as pioneered by K 919.

The primary reason why the ten KAs were not assembled at Hillside was that there was no way of transporting complete locomotives between the North and South Islands at the time (the first inter-island rail ferry did not commence until 1962).

Vulcan Foundry of the United Kingdom supplied parts for fifteen locomotives, including most chassis components, tender bogies, and boiler foundation rings.

The General Casting Corporation of Pennsylvania, United States supplied trailing bogie and rear end framing.

Nineteen locomotives were built between 1939 and 1941, but wartime circumstances meant construction of the remaining sixteen lasted from 1941 to 1950, a period much longer than NZR management anticipated.

The shrouding, while cleaning up the appearance of the locomotives, was open at the top and began gathering soot and dust that affected the working environment in the cab.

After the war, a coal shortage also occurred and NZR decided to convert a large number of locomotives to oil burning.

On the last day of the steam power on the Limited, in April 1963 and the Express in February 1965, KA locomotives, worked the train right through to Auckland.

[6] The southern section of the NIMT from Otaki to Marton junction is almost as easy graded and fast as most of the Main North Line.

At one stage, KA 944 was sent to the South Island for an overhaul at Hillside Workshops and for subsequent use on the Midland Line along with the KB class.

Due to the short nature of the Silver Stream Railway, 935 has been converted from superheating to a saturated state by removal of the superheater elements.

KA 942 was preserved by Ian Welch in 1972, after having been laid up at Hutt Workshops as a possible addition to three K class locomotives being used as a stationary boiler supply.

In 1989 it was moved to the Glenbrook Vintage Railway where it was restored to working order, and mainline certified - first running from July 1990, wearing its former streamline shrouding, being used on an excursion in the South Island in 1992.

[11] It then pulled the North Island Main Trunk Centennial in April 1985 with Glenbrook Vintage Railway's JA 1250 "Diana".