The Southern Region ran the majority of its BR MkI coach stock in fixed sets, as did to a certain extent Network SouthEast for services on former Southern lines.

When first introduced on the Southern Region MkIs carried the prevailing crimson and cream livery. The last BR(S) stock appeared in these colours in 1956 .  From then onwards coaches were constructed or repainted into the Southern's beloved green - albeit to a different richer and darker shade than SR malachite.  The original BR livery specification was for black coach ends but from 1964 onwards ends were painted the same colour as the body sides to take advantage of the new airless spraying techniques. From the late 1960s BR Corporate blue and grey prevailed, though it is rumoured the Southern Region took the BR blue/grey specification being for main line trains only too literally and turned out a small number of coaches for a short time in overall blue.  From the late 1980s the colourful and complex Network SouthEast livery was applied.

The Southern Region introduced continental style colour bands at cantrail level, initially on boat train services, in 1960. Yellow designated first class accommodation and red catering vehicles. This soon spread to other Southern Region coaching stock, including Maunsell, Bulleid and EMU designs. By 1963 this practice had spread to the rest of BR.

Although second class accommodation was generally abolished in the south in stages up to September 1923 the Southern Railway and Region retained it for use on continental boat train services. Fifteen MkI second class opens were specially constructed for Southern boat train services. Passengers were seated in eighth bays of 2+1 pattern with a central door on each side opening into a seating bay. When European second class was abolished on 3 June 1956 the former third class was redesignated second and these fifteen unique coaches were initially designated second without any special markings. However their 48 seat only capacity created rostering problems on intensive Southern services so in 1960 they were reassigned and relettered as Restaurant Cars until an early withdrawal.

Even though international passenger travel by boats had been largely killed by airliners the Southern still continued with its traditional Ocean Liner Boat Train service to Southampton, though in its later days concentrating on cruise liners and the remaining intermittent Trans-Atlantic services. However on 1 November 1991 the last Ocean Liner set consisting of eight MkI FKs and one MkI BSK was disbanded to provide additional stock for West of England services.

Following Southern Railway practice the Southern Region stabled its coach stock at various points throughout their system though with the progressive withdrawal of locomotive hauled services all hauled coaches were allocated to Clapham Yard by 12 May 1986 (excepting six BSKs for the remaining SE/Central newspaper services). Further rationalisation of hauled coach stock under Network South East saw stock for the few remaining Southern services being supplied by (horror of horrors to Southern enthusiasts!) Old Oak Common and Newton Abbot.

For more information on BR MkI coaches you are strongly advised to read the master work on this subject - the book "British Railways Mark 1 Coaches by Keith Parkin MA, published by Pendragon in association with the HMRS, ISBN 1-899816-07-0".

Following pictures of MkI coaches in BR(S) green are from the RCS "Golden Jubilee" set and in their current specification are not authentic for BR(S) service. Coaches from this set may not have been allocated to the Southern Region at any time and they now run on Commonwealth bogies whereas B1 bogies would have been used at least in early BR(S) service.