While many suburban coaches were built in sets the GWR also built some
'loose' coaches which could be used to strengthen (lengthen) trains at
times of peak demand, or be formed into extra trains when required. The
production of the brake third and composite coaches from the 4-coach
sets as separate coaches permits other formations to be created, like a
2-coach brake third + composite formation, either as a separate train or
an add-on to a set.