GER B12/3 4-6-0 No 61575 from Cambridge shed with a Leicester-Clacton relief approaching Colchester on 2 August 1958.
The Leicester-Clacton had a varied route, and it isn't necessarily the case that a relief would follow the same route as the timetabled train.

But 1958 was the last year that the Kettering-Cambridge line was still open: the timetabled train was booked that way and I suspect that a relief would go that way too. Therefore 61575 probably worked from Cambridge.

Photo: K.L. Cook / Rail Archive Stephenson