The class was preceded on the WAGR system by the M/Ms class Garratts. The class were used extensively on WAGR lines with light rails and sharp curves, as a consequence many of the smaller older branch lines on the Darling Scarp; as well as those with steep inclines such as those on the Mundaring Weir, Nannup, and Flinders Bay lines. In their later years, the boiler pressure was reduced to match that of the M/Ms class. By this stage they had been concentrated on the Bunbury to Boyup Brook and Pinjarra to Boddington lines.
Hearsay evidence suggests that the poor quality of the boilers in the Msa rendered some inoperable by the late 1940s. The last remaining Msa was awaiting moving to a preservation status in the adjacent to the Midland Railway Workshops, when instructions were misunderstood by a scrap metal company employee, and it was cut up.