Second class coaches generally form over half of any train, the lower priced seating generally 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.