An SECR O1 class 0-6-0 in the 1920s. The presence of the conductor rail indicates early 'twenties or later, assuming it's an SECR line. We think this is a down Victoria to Maidstone East or Ashford service at Sydenham Hill, probably around the time of electrification to Orpington and grouping in 1923/4

Photo: Rail Archive Stephenson