Annie Mole's, webmaster of Going Underground, daily web log (blog).
If you like this you'll LURVE One Stop Short of Barking, THE fun and informative BOOK about travelling on the London Underground.
The carriage would have been filled with lots of lovely pine smells. Great way to mask out the normally damp coat / sweaty / smoky Underground odours of the day. Some commuters may have left their journey with pine needles on their hats and coats, but that's a small price to pay for some great festive spirit.
Even with the image enlarged I can't work out which line this is. Maybe someone would like to hazard a guess.
Thanks to Georges P for bringing this to my attention who originally saw this on Vintage everyday.
"The magic, mystery & sometimes maddening shortcomings of London's Tube are documented with love, enthusiasm & sometimes despair by its unofficial social historian." The Guardian