During 1963 and 1964, the railways were plagued with derailments. There was such media attention on the events that in 1964 a derailment per day was being recorded, though some of the events were very minor. Others were spectacular. One derailment near Nowa Nowa, left zig zagged derailed vehicles laying across the top of the Stoney Creek bridge, fifty feet above the ground.
One of the derailment causes was found to be thee "W" guards of _IZ_ / _GZ_ / _HZ_ type (27 ton capacity) wagons, which were found to be cracking where they were rivetted to the underframe. To rectify the problem the "W" guards were replaced and the wagons derated to 22 ton capacity. These wagon classes were recoded to RY with the original numbers kept.
The combined IZ / GZ / HZ number group was 1 to 2000. With conversions to other wagons from the mid 1950's and scrappings due to derailments, there were many gaps in the number group. Lowest numbered RY was 'No.2'.
The conversion of all the wagons took about four years. While this was progressing the railways began converting the outside sill versions of IZ / HZ / _HY_ / RY to _GY_ wagons. Apart from the rebuilt wagons coded HY, the IZ / HZ / RY conversions involved removing the old rivetted superstructure and building a welded one.
On notable wagon was RY 600, built 1931 as IZ 600. It has been billed as first "all welded" in the world. Welded bogie wagons were built in part from the late 1920's in North America and for Europe, wood and rivet construction were normal. Perhaps "all welded four wheel wagon" was more applicable.
"Outside sill" wagons ( as photo above ) were numbered in the range 1 - 599, 761 - 810. Other wagons were standard channel underframe of either wheel ( transverse ) or lever handbrake.