By about 1978, the _ZP_ vans saw little traffic. They were originally used as transition vehicles between autocoupled locomotives and screw coupled carriages from the mid 1950s with the introduction of diesel locomotives replacing steam engines fitted with dual screw/auto couplings.
For conversion to ZD ( D for dual coupled ), the rail ballast was removed, the code letters altered but with the dual couplings retained.
The ZD numbers were: 19, 31, 211, 237, 239, 260, 468 - 471, 494, 500, 600, 604, 611, 619.