The final Mark 2 carriage was departmental 999550, in 1977. it is still in service with Network Rail as a Track Recording Coach. The later versions (2D onwards) look somewhat similar to the later Mark 3 design. The Mark 3 is longer (75 feet as opposed to 64 feet 6 inches), has a large skirting between the bogies to conceal the ancillary equipment, and has a ridged roof as opposed to the smooth roof of the Mark 2. The development of the High Speed Train overlapped with that of the final production run, and the Mark 2F "previewed" many features incorporated into the Mark 3, such as new seating, plastic interior panelling, and floor-sensor-operated automatic gangway doors.