In 1899 the railways placed a large bogie open wagon into coal traffic service. The code was OO and it was number 1. Expected traffic was hauling coal from Wonthaggi to Melbourne. At the time it was billed as the largest bogie vehicle '..in the British Empire..'.
It featured 6 wheel plateframe bogies and had a capacity of 40 tons. Large indeed for a railway that was running mainly 10 ton capacity wagons. With double doors on each side at each end, diagrams also show it with trap doors in the floor near the transverse centreline.
It seems the wagon failed in service as unloading was extremely difficult.
And this is where the mystery starts: the tests were not successful, yet five more entered service two years later in 1902. The 1902 Annual Report indicates the the five vehicles as under construction. The writer believes the wagons were built as a batch and when the first vehicle failed, the others remained in storage until suitable traffic was found for them.
Photographs show these vehicles on the coal stage at North Melbourne Loco Depot. It seems as though the class was used for loco coal traffic only and never entered continuous revenue service.
There is also a photo showing one of these wagons loaded with bags of wheat.
From 1912 the vehicles were drastically modified with extended sides and corrugated roof. They were used as breakdown vans which housed all the equipment required to rerail locomotives and rolling stock. These vans accompanied the steam cranes to accident and derailment sites. These vans helped replace older breakdown vans coded _S__. The 'vans' retained their original class and numbers until the recoding in 1956 to _HH_.