To improve the riding qualities of fixed wheel brakevans, twenty vans were 'ballasted' with three tons of rail. The rail was placed along the floor of the van area and a new floor placed over the top.
Vans of the type _Z__ / _ZL_ type were modified in 1971 and reclassed to ZB, numbers 1 to 20.
The first scrapping was in 1973, with most of the class removed from service in the mid 1980's.