Five horseboxes formed part of the original M.R.W.A. stock. They are believed to have been identical in design to the four built for the G.S.R. which later became part of W.A.G.R. class A. They were built by the Lancaster Wagon Co. It is not known what the original classification was. At some point between 1902 and 1911 these five horseboxes were renumbered 255-259 and became class H after the last old ballast wagon (29H) was reclassified IA. They were all rebuilt as class HM before 1927 (probably 4 of them in 1924). In 1932 they were again renumbered HM71-75. They were all out of service by 1934 but still in stock when one was used to produce BM803. Another provided parts for CA718 in 1935 whilst the last three became DXA613-615 in 1936. It is probable that only the chassis was reused in these rebuilds.

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The outline diagram above is the WAGR diagram for the GSR horsebox, believed to be identical to the MRWA H class.