Built from 1972 through until 1976 this 40ft long steel carrying wagon was a radical departure for British Rail after a 10 year break in building wagons for the conveyance of steel. With the deep under-frame, corrugated steel floor to dissipate the heat of hot steel loads and to aid loading operations, air brakes and a brand new bogie design, these were wagons to herald a new age. The first lot preceded TOPS and was lettered Steel AB, later gaining the TOPS code BAB due to their through vacuum pipe, later lots without vacuum pipes were lettered BAA, a code the first batch would eventually carry as their vacuum pipes were removed over the next 20 years.

Possibly the easiest way of telling the different lots apart are the changes in design of the wagon ends. Originally a high end formed of five uprights was fitted, replaced on later batches with a lower version with four uprights and no bridging section between the centre pair this too was modified later on with additional sections to raise the height. The final batch gained an end design similar to that of the larger BBA wagons. All wagons were given a variety of sockets to fit additional stanchions to control the load and could be found with high ones fitted to contain billets and small pegs when loaded with 'eye to the sky' coil loads. Many wagons were fitted with coil cradles to help avoid damage to the load, both transverse and in-line alignments were used and this saw the early wagons having their ends modified with a gas-axe to aid loading operations when in-line cradles were fitted.