Carrying a 14A Cricklewood shed plate and passing a LNER mile post. The negs immediately before this on the strip were near West Wycombe so it is safe to assume this is somewhere on the GW & GC Joint railway.
Possibly a Nottingham Vic - Marylebone service on the GCR mainline. The telegraph poles with larger top spar were evident in Calvert area.

Richard Pike says "Yes I'd say crossing the Oxford - Cambridge line at Calvert. There was a colourlight distant behind the train that would have needed the power wires on the extended arms on the pole"