The MK3 coaches entered service between 1975 and 1988. These coaches were originally designed as Locomotive hauled stock but were later modified for HST use. The MK3s share lots of similarities to the MK2s but had a rigid roof and skirted under frame, the length was nearly 10 feet longer at 75 feet over the buffers and the body shell was designed with a stressed steel skin which was lighter and stronger than previous designs.
One of the main advances on the MK3 from the MK2 was the secondary air suspension fitted to the bogie system. This allowed for smoother running and with the primary coil springs and hydraulic dampers the coaches were able to run smoothly at 125mph instead of the limited 100mph from the MK2s.
Most MK3 coaches built are still in service today, including on the Royal Train.