David Higgs explains what is going on "The 3F tank 47521 is actually the Bletchley Station Pilot / Carriage Shunting Engine. 47521 is on the "Down Slow" Line and is backing the Empty Coaching Stock (off an Ex. Euston Local Train) into the "Middle Long Siding" which is just beyond Bletchley No. 2 Signal Box between the Down Slow and Up Fast Lines. The Lamps on the Loco consist of One Red and One White Lamp showing it's on Shunting Duties.

As a 10 year old lad I was privileged to be allowed to occasionally ride on the cab of the Blecthley Pilot Engine (47500 or 47521) whilst carrying the same duties as in your photograph"