Only three classifications in the I group have ever been used. In 1900 all of the existing four wheeled flat or single bolster wagons were grouped together under this code. Before 1900 all of these wagons would have been part of class M.

The largest group were the two hundred or so single bolster wagons built to the N.Z.R. type N design from 1893-4. These wagons were 11'-0" long over headstocks and had a single swivel bolster. In 1908/9 a total of 15 of these wagons were up-rated to 10 tons and reclassified IX.

By 1941 all but five of these wagons had been written off or retired to Jetty stock. However, in 1942, some of the Jetty stock wagons were returned to service. The bolsters were removed from the survivors in 1948 and many spent their final years as shunter's trucks. The last survivor was written off in 1972.

The second largest group was the wagons inherited from the G.S.R in 1896. They may have been of two types. One group of eighteen wagons was 12'0" long over headstocks. They are believed to have been eliminated by 1935. The M.R.W.A. also had wagons of this type in their class E but these had gone by the 1950's. A second group of twenty-five 15'-0" long wagons appear to have disappeared before 1904.

There was another group of wagons on the old W.A.G.R. chassis and a few odd wagons from other sources some of which did not have bolsters. All of these odd wagons had been eliminated by 1903 and so tracing their history is difficult.

Between 1920 and 1922 a total of 75 standard I class wagons had their bolster removed and small ends added to allow them to carry a pair of coal boxes. These wagons (class IM) were gradually withdrawn during the late twenties and early thirties and the last group were written off in 1935.