On southbound Mobil empties David J Hayes adds: Although the information below post-dates this image, it might give a clue as to what the service is. As of May 1974 train 6O68 was shown as the 10:35 SuX empties from Thame to Hoo Jn., which was booked through Kensington Olympia at 12:20 on a Saturday (the day of this picture) and at 12:15 Mon-Fri. A crew change was booked to take place at Hither Green station.

Since my first comment, further WTT information has come to light, which might provide an even more accurate identification for this Saturday working. I believe the loco was Stratford-based at the time of this image and the 6O68 headcode, as of May 1971 (thus post dating this image), shows the service running as the 11:17 SO Thames Haven to Salfords (arr. 3:14pm), which was booked through Kensington Olympia at 1:53pm (the angle of the light would probably be about right). The service departed at the earlier time of 10:45 during the week, but with the same 1:53pm passing time at "Kenny-O".

Salfords is south of Redhill on the Brighton line. The terminal supplied aviation fuel to Gatwick airport. There was also a stone terminal located there as well.