The Mark 1 coaches were built in two distinct tranches: the early vehicles (1951–1960) and, from 1961 onwards, the "Commonwealth" stock – so named on account of their bogies, which were a variant of the bogie designed by the General Steel Castings Corporation (formerly named the Commonwealth Steel Company) of Granite City, United States of America.

These lengths allowed for compartments or seating bays 6 ft 3 in (1.91 m) wide, plus space for toilets and entrance vestibules; a typical design of Mark 1 vehicle, the TO (Open Third class), had eight seating bays, three entrance vestibules and a pair of toilets at one end.

An unusual feature of the design was the bodyside tumblehome curvature, of 28 ft (8.53 m) radius and just noticeable; the windows had flat glass and consequently the upper quarter was separate and in a different plane from the lower glazing, with an intervening transom, and the steel panels were recessed on either side of each window opening to accommodate the difference between the flat glass and the curved sides.

The original vehicles had timber veneer interior finishes, and on the main line vehicles small plastic labels were fixed to the panels, giving the type of wood and its country of origin e.g. "Crown Elm Great Britain"; "Lacewood Great Britain", etc.

In 1955, an order was placed for 14 vehicles, with manufacturers being invited to incorporate innovative features; perhaps the most striking of these prototypes, which were completed in 1957, were those constructed by Cravens.

Laminates were used instead of timber panelling, and in the last of the Mark 1 hauled vehicles, fluorescent lighting was fitted instead of tungsten bulbs.

The original bogies were a double bolster type, which like the carriages mounted upon them, were designated "BR Mark 1" (BR1 for short).

An important factor was the compressive coupling, which provided excellent inter-vehicular damping through the gangway end-plates, which quickly became highly polished, indicating that they were performing this task.

None remain in front-line service, and very few are preserved due to asbestos insulation being present in most carriages, which was very difficult to have professionally removed.

The vehicle, numbered S1000S, was mounted on the underframe of Mark 1 Tourist Second Open S4378, which was written off as a result of the Lewisham rail crash in 1957.

The improvement in safety the Mark 1 thus represented was praised by the Chief Inspecting Officer of Railways, Lt Col I.K.A.

McNaughton (Chief Inspecting Officer of Railways, Department for Transport), in the Sir Seymour Biscoe Tritton Lecture to the Institution of Mechanical Engineers in 1977.

Speaking of the fall in fatalities since 1955, he put forward his opinion that a major factor in this improvement was "the introduction in 1951 of the BR standard Mark 1 passenger carriage, which, over a period of about ten years, replaced pre-war designed rolling stock on most principal routes.

The 1988 Clapham Junction rail accident highlighted that by the 1980s the Mark 1 coach was dated, and less able to withstand collisions than newer designs.

"[14] British Rail was still using some 4EPB and 2EPB (classes 415 and 416) multiple units with underframes that had been constructed before World War II and these were a priority for replacement.

[15] A proposed modification to extend mainline use beyond 2002 at the time of the 1999 HSE instruction was 'cup and cone', however trials were inconclusive and deemed expensive in comparison with the safety benefits.

In October 2002 the Health and Safety Executive extended the permitted use of Mark 1 based rolling stock until 31 December 2004 with the proviso they be fitted with a Train Protection & Warning System.

When British Rail was privatised in the mid-1990s, only Connex South Eastern was given an extended franchise (15 years) in return for ordering new trains.

A similar story took place in Scotland, with the last of ScotRail's Mark 1 based Class 303 multiple units surviving until December 2002.

The postal train services on the West Coast Main Line were worked using new Class 325 multiple units.

However, these units proved to be extremely troublesome in service and due to technical failures were often loco-hauled instead of running under their own power [citation needed], and therefore Mark 1 formations continued to be used here, and elsewhere on the network.

The conditions usually involve the fitting of central door locking and having either a non-passenger or non-Mark 1 carriage at the ends of the train.

[19][20] South West Trains was granted a derogation to continue to operating two three-car 421s on the Lymington branch line until 2010.

The long service life of Mark 1 carriages means that a heritage railway can (potentially) recreate an authentic period train of the 1950s, 1960s, 1970s, 1980s or early 1990s, when painted in a suitable livery.

When British Rail began to run down its overnight sleeper trains, many heritage railways replaced them with newer Mark 3 cars.

Since most Mark 1 sleeping cars had been built with asbestos insulation, which was expensive to have decontaminated professionally, most were sold for scrap.

1962 saw Southern Region adopt the now familiar yellow cantrail above first class and red above dining cars.

During the 1980s a complete rake of 1950s built corridor compartment second class Mark 1s (including a BSK) which operated exclusively on the Glasgow – Stranraer route and connected with the Irish Sea ferries, were painted into a very striking Sealink livery of red, blue, and white, and internally the vestibules (but not the main side corridors) were painted bright yellow to match the refurbished EMUs of the period.