In 1917, two tank locomotives were built in the Addington workshops with boilers that were originally intended for class AB Pacifics. These now had an additional trailing axle and many common parts with the Pacific, which simplified maintenance. Two almost identical variants were built in series, which were designated as WAB for use on main lines and as WS for use in suburban traffic. Between 1947 and 1957, some WAB were replaced by other locomotives, after which eleven were rebuilt into class AB tender locomotives. In the sixties the remaining WAB were retired. 