...if not perhaps a reader can explain its point?
UPDATE: This from a Mr Peters...
You asked if 'Culham ?' was a spoof.
I expect it was found near the Turbo stop mark on the platform.
If so it is a 'cheap and cheerful' way to remind drivers to check if they should be calling at Culham (the next station towards Oxford), as most trains do not and this can help to reduce the risk of an erroneous 'fail to stop'.
I have seen something similar (but more permanent) at Bache on Merseyrail, reminding drivers to check if they call at Capenhurst as only half of the trains on that route do.
UPDATE: This from Chionanthus Virginicus, evidently in playful mood...
Can we hope for a similar poster (below) to be displayed outside Marsham Street?
Of course having blinkers rather than cabside windows may explain DfT's apparent tunnel vision.