Second class coaches generally form over half of any train, the lower priced seating�usually being choosen for private and leisure journeys. Typical 'model railway' sized trains might be formed as follows
4 coaches 2 x SO or SK + BSK + CK
5 coaches 2 x SO or SK + BSK + CK + SO or SK
Both of the above 4 and 5 coach train formations were used during the 1980s on Cardiff to Portsmouth and Cardiff to Crewe/Manchester services.
Coach Codes used to quickly identify coach types
SO - Second class Open plan coach with seats set around a table and a central corridor.
SK - Second class side corridor coach with seating compartments. C is used to indicate a Composite coach (below) so K is used to indicate the side corridor.
CK - Composite (first and second class compartments) side corridor coach.
BSK - Brake Second class side Corridor coach, contains passenger compartments at one end and a luggage van at the other with an office for the guard in the middle.