The Salmon is a flat wagon, measuring 62’ 0” over headstocks for carrying 60’ 0” lengths of rail or track panels. They have also been used without bolsters to carry bridge girders etc. The design was based on an LMS wagon (BBP/Borail J/MD) some of which were revenue stock, including the BR period. The first batch of wagons (162) was built to the LMS Diagram 19, with wood floor and LMS Bogies (This is available as C47). The rest of the Salmons featured a steel floor, and GWR style plate bogies. The original bogies were (Kit C46) standard 5’ 6” wheelbase (Nos. DB 996100-151/ 214-362) , and (Kit C27) 8’ 0” wheelbase (Nos. DB 996363-677, DB 996804-969/996995-7019), a type only found elsewhere on the “Borail E”. Some have low side rails, which may be wood or steel, and others have longitudinal timbers between the bolsters. Bolsters are not always fitted, and the wagons without them are used for carrying one to four (five, if timber sleepers) recovered track panels, or new concrete sleepers, closely spaced in three rows with timber spacers between each row. About 300 Salmon wagons were fitted with ASF bogies from 1998, enabling them to run at higher speeds, some as high as 75mph.

In mid-2009, work began on fitting guides to locate track panels, consisting of two substantial tubular frames at each end, plus two smaller stanchions in the centre. These, and the headstocks, are bright yellow, but the rest of the wagon remained in the “livery” it had already however tatty it was. About 90 wagons have had this done and the code is YKA Osprey.