The design for the Fishkind-coded ‘Sealion’ wagons was based on a 1903 design by the London and South Western Railway. A typical hopper wagon would have slab sides with seven vertical ribs and three discharge chutes. These chutes were situated at the outside of each rail and in the centre. The end of the wagon featured three handwheels to control the chute doors.
A total of 128 wagons were constructed at Shildon Works in 1971 and 100 members of the batch were fitted with dual brakes. AFI vacuum brakes allowed for faster emergency braking. These dual brakes meant that at one end of the wagon, there was a small air brake cylinder, and at the other end were two larger cylinders for the vacuum brakes. Under BR TOPS, the wagon’s unique combination of brakes meant that it was coded YGH and as the Fishkind ‘Sealion’. A further 360 ‘Sealion’ wagons were constructed at Shildon Works between 1972 and 1974.