Birmingham suburban services, at least on the Western Region in the 1950s were frequently composed of one suburban brake in between a second and composite coach. Additional coaches would be added at peak times.
As others have said; you could see trains composed of BR Crimson and BR Maroon coaches. There would be a schedule for maintenance and repainting and each coach would have to wait for its turn to visit the paint shop.
Mainline coaches, like the Stanier ones, could be seen on minor routes behind a tank engine as part of a through service that was attached to, or detached from an express. For example, on my local line, trains from London to Birmingham might include a couple of coaches for Stratford-upon-Avon, which were detached at Leamington Spa (General).