David J Hayes says "
This to me looks like an evening shot looking into the setting sun. Despite the headcode being displayed, the service, therefore, could be 4V64, the 20:35* SSuX Willesden to Swansea (Danygraig) 'liner, which was booked to run-round at Kensington Olympia (20:50-21:05). The service had previously departed at the later time of 23:00 off Willesden. In later years, the run-round manoeuvre was performed at Mitre Bridge Junction, the train then travelling via Willesden South West Sidings and Acton Wells Junction.

(*Also shown as departing at 20:20 in another WTT for the same period)"