Peter Kerslake says "Rather unusually 6847 "Tidmarsh Grange" is at the head of a special working, denoted by the " Z 72" Reporing Number and it could possibly be a troop train, perhaps ?
Allocated a few years earlier to Newport Depot she's a local engine, possibly still shedded at Newport, and her tender is piled high so she does not appear to have come very far since taking this working".