The Class 121s took over services originally monopolised by GWR-built railcars, but as more and more cross country and branch lines closed that work became less and less. Consequently, the Class 121s were concentrated around the Reading, Bristol, Cardiff and Plymouth areas. They were used on the West London branch lines, the Severn Beach branch, lines in the Welsh valleys and the Cornish branch lines. For the first 25 years of service the units were concentrated solely on the Western Region. They seated 65 people in two saloons in 2+3 seating, 45 seats in a saloon behind the cab, with a doored partition dividing it from a 20 seat saloon, a further door dividing it from the guard and luggage van. The standard power train was originally fitted with AEC engines, but these were replaced in later years with the Leyland 680. The units have also proved to be useful as route learning vehicles and/or Sandite units.

Photo by: Matthew Smith