The railways built 350 20 ton open wagons during 1925/1926. These were the first of the _I__ wagons to break the 16 ton load capacity. They were built as I wagons and were recoded to IY in 1929 as part of a project to segregate I wagons into capacity groups. The wagon numbers were 15220 - 15569.
Capacity was raised from 20 tons to 22 tons from late 1929.
They were originally fitted with stays between the "W" guards ( axle box supports ) and the underframe. It seems that these stays remained until the mid 1950's, possibly removed at the same time as buffers.
The class remained intact until the mid 1950's when conversions to specialised wagons began.
In 1948, some non grain proof wagons were issued to service with the IY code. Possibly to segregate the rivetted wagons from the new welded wagons, these new wagons introduced in 1948 were recoded to _IO_. As the 'IO' code proved unsuitable for service, the wagons were relettered once again, this time to _HY_. These wagons were numbered in the group from 15870 onwards.