This caused wear and cutting of the valve and port faces with resultant steam leaks.

In 1929, it was decided to not to superheat any more slide valve engines and those so fitted reverted to saturated steam as boiler renewals became necessary.

The engines that survived until the last decade of steam were mainly used for heavy shunting in yards such as Rockhampton where the loads were beyond the capacity of the PB15 class.

All, except the class leader, had these replaced by a “standard” Baldwin style sandbox on their boilers.

Association of American Railroads Master Mechanics smokeboxes commenced being fitted to members of the class in 1945.