David J. Hayes says The WTT shows 1S31 as the 19:19 MWSO Newhaven Harbour to Stirling Motorail service, which ran during the period 19th May to 27th September 1969. It called at Kensington Olympia for a crew change (20:51-21:00) and was booked via Old Oak Common rather than the WCML. Curiously, the date of this picture is a Friday, which was one of the days 1S31 wasn't booked to run although there may well be a WTT supplement showing otherwise.

The routing of 1S31 from London, it seems, was actually by way of the Midland Main Line, which entailed a convoluted meandering from Kensington Olympia of via Old Oak East Junction-Park Royal-South Greenford-West Ealing-Acton Main Line-Acton Wells Junction and Brent Curve Junction".