The JAs were primarily express and express freight engines running from Invercargill to Christchurch but extended their duties during their brief prime on the South Island Main Trunk from Bluff to Parnassus in the north, and on the Midland Line as far west as Arthur's Pass. Before the full delivery of the JA class, English Electric diesel locomotives had already taken over the main freight services and summer expresses on the Dunedin-Oamaru section. The JA was too big for long branch lines like Otago Central Railway or Kingston Branch. Despite some restrictions, the class were the mainstay of the South Island network.
