The units 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 was 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.
Eric Stuart notes: "at one time one use was to add on to Cross-Country units working the Newport - Shrewsbury - Crewe line. They were used to boost the available power. I recall that they were locked and had signs on the windows stating that they could only be used if specifically authorised by the Guard (presumably if the main unit was packed). One could see trains of 2 x CC units, each with its 121 - the CCs in blue and grey and the 121 in blue!"
W55034 on the Bridport branch, working the 10:35 from Maiden Newton in April 1975. Robert Frise Collection
In 1986 three, nos. 55032, 33 & 34 were transferred to Tyseley, now under Midland Region control, and used on local services around Birmingham.
The next three images by Mike Cooper were taken at Paddington in January 1988. First, two 121s added to a 3-car 101, and then two 121s added to a 3-car high density set. Finally a trio of 121s with a 2-car 101.