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.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).