The Taunton sleeper train fire in 1978 highlighted a number of deficiencies in the ageing Mk1 sleeping car design, prompting British Rail to replace the sleeping car fleet with a new coach based on the Mk.3 coach bodyshell design. Originally 236 were ordered, but reducing demand for sleeping car berths and the projected improved reliability of the new coaches saw the the total reduced to 207. This also proved excessive as improving line speeds and deployment of HST trains on more routes steadily reduced journey times, providing passengers with daytime services at timings which suited their travel plans.
The new mk3 sleeper coaches were a definite upgrade from the mk1 design with modern features, air conditioning, retention toilets and alarm systems designed to wake sleeping passengers in an emergency. The BT10 truck introduced with the mk3 coach family has proved to be one of the finest riding trucks ever placed in service, making the entire interior much quieter, even on jointed track. Two interior layouts were produced, a full sleeper (88 coaches, code SLE - SLeeper Either class) with 13 compartments and a car with one compartment equipped with a pantry for the car attendant in place of one sleeping compartment (120 coaches, code SLEP - SLeeper Either class Pantry). A small number have also been converted to provide improved access for disabled passengers, coded SLED (SLeeper Either class Disabled). All compartments were equipped with fold-down upper bunks, allowing the compartment to be configured for a single occupant as first class or two occupants as second class. The provision of one attendant sufficed for 2 coaches, with an attendant call system connected between the coaches.
The excess number of coaches available for the reducing sleeper services resulted in a number of these quite new coaches being redeployed by the mid-1980s, with some being sold to European railway operators for sleeper services and heritage railways as volunteer accommodations. Conversion of one sleeping car to a day coach was started, though the work required quickly proved excessive and too costly to be viable. Several were more successfully converted as generator vans for the aborted Nightstar service and for use as mess and sleeper vehicles for engineering staff working at remote sites.
Current operators of the mk3 sleeping coaches in passenger service are Great Western Railway for the Night Riviera service between London Paddington and Penzance and Serco for the Caledonian Sleeper service between London Euston to Glasgow, Edinburgh, Aberdeen, Inverness and Fort William.