In-order to allow running at 125 mph on Britain's railways, new rolling stock was needed by British Railways to replace the aging Mk2s. Significant improvements included new secondary air suspension between the bogies and the coach body as well as aerodynamic skirting on the underframe. Mk3 Coaches are 75ft (23m) long enabling far greater capacity than older coaches. Mk3 Coaches also incorporate disk brakes and wheel slip protection enabling faster deceleration.
The first Mk3 Coaches to be delivered were used as part of the HST prototype along with the two Class 41 diesel power cars in 1972. Mk3 coaches entered service in 1975 along with the Class 43 forming the iconic InterCity 125 trainset.
Mk3 Coaches continue to be a mainstay on British railways in the 21st century despite their increasing age. With the introduction of the new Class 800 and 802 IEP units on the Great Western Mainline, GWR cascaded remaining IC125 sets to rural routes, shortening them to 4 passenger coach 'Castle' sets. These Mk3s are also updated to comply with new regulations mandating considerations such as improved toilet facilities, disabled access and automatic doors.