Pullmn 1st class parlour car Rosemary on the storage road between platforms 2 & 3 at East Croydon station on 6/6/53 which was the date that the Dery Day royal train passed through. We were all aware tht there was at least one stand-by engine for those diagrams but it seems that supporting Pullmans were also catered for.
However, David Woodcock suggests that It is possible that she was just en route to/from Preston Park Pullman works and was awaiting transfer from/to a Stewarts Lane trip working as she was a Bournemouth Belle car at the time. However, I have also come across an interesting photo which depicts her, around 1.05am on an unknown date, in platform 3 at Waterloo, where I suspect she is being used for filming, possibly, judging by the numbers present, for a scene in a feature film, and it could just be that your shot depicts her being moved from Preston Park in connection with that as obviously a newly overhauled car would have been chosen.
[Mike Morant collection]