I believe they did that "in house". They added the cab to an MT4.
WW uses a pair of GP9/10s on the WST, and until very recently, used two GP9/10s spliced by a yellow ex CR MT4 slug known affectionately as "the banana" on the Sandman. In an excellent display of shortline resourcefulness, MT4 120 was taken to the shop at Gore, where in a Frankenstein-like fashion, she will soon be "alive." The shop forces have obtained a cab from a retired IHB SW1500, and are currently in the process of turning 120 into a fully functioning road slug, so that one geep and the slug can power the Sandman to save fuel. GP10 572 (ex CR 7572) is the 120's mother, so this pair will be the Sandman power in the coming years. For now, a pair of geeps powers the Sandman.from: link