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.