Approaching Kennington Junction, Oxford, from Princes Risborough after crossing the three span bridge over the River Thames. Matt Pinto points out that it is not Railcar is number 15 (as originally captioned). If you look at the shape of the valences, it is one of the Park Royal bodied railcars (2-4). I have seen a photo of No4 at Oxford before so it may be this one however the condition of No2 used post war regularly on the Thame trains matches perfectly.

Peter Kerslake adds "With frames built at Swndon Works and with running gear and power units from A.E.C.at Southall there would appear to have been some form of collaboration between Swindon , Park Royal and the Gloucester Railway Carriage Company when it came to building the bodywork of the former Great Western Railcars with examples coming from each!
Railcar number 4 seen here was apparently a Park Royal build and the last of the three introduced in 1934 for running between Cardiff and Birmingham, all fitted with a mini buffet, the units later being moved to various Depots such as Weymouth, Reading and Newbury when their popularity on the Birmingham services exceeded their seating capacities.
This Railcar ended her days at Newport Pill , running for some considerable length of time on the services from Newport to Monmouth before being withdrawn in the late 'fifties and restored by Swindon for preservation in 1962."