A Brake Standard Open or BSO, is a type of railway carriage used by British Rail. Both Mark 1 and Mark 2 types were built. Each consists of a standard class open passenger saloon with a centre aisle, a guard's compartment with hand brake and a lockable luggage compartment.

British Rail Brake Standard Open
In service	1955–
Manufacturer	Mark 1: BR Doncaster, Wolverton and Gloucester RCW Mark 2: BR Derby
Family name	British Railways Mark 1, British Rail Mark 2
Constructed	Mark 1: 1955–1963 Mark 2: 1966–1974
Number built	340
Fleet numbers	9200–9539
Capacity	Mark 1: 39 Mark 2/A–D: 31 Mark 2E/F: 32
Operators	British Rail
Specifications
Car length	66 ft 0 in (20.12 m)
Width	9 ft 3 in (2.82 m)
Height	12 ft 9+1⁄2 in (3.90 m)
Maximum speed	90–100 mph (145–161 km/h)
Weight	33 long tons (34 t; 37 short tons)
HVAC	Steam, Electric or both
Bogies	BR1, Commonwealth, or B4
Braking system(s)	Vacuum, Air, or both
Coupling system	Drop-head knuckle coupler on draw-hook
Track gauge	4 ft 8+1⁄2 in (1,435 mm) standard gauge

A number of Mark 1 and Mark 2 BSOs were converted to Brake Standard Open (Micro-Buffet) (BSOT), and fourteen Mark 2 BSOs were converted to Driving Brake Standard Open (DBSO). A number of BSO and BSOT coaches have been preserved, and some are still in use on main line charters. One significant survivor is No. 9267, the last surviving coach in the final excursion train on the Somerset & Dorset line on 6 March 1966, which is appropriately located at Midsomer Norton.^[1]