I particularly like the way that no one seems particularly interested in them, as though you see people on playing board games on the Tube every day.
But perhaps you do? In all my years of Tube travel I can't say I've seen anyone playing board games. Loads of Sudoku, crosswords, word search, which are all understandable and usually fairly solitary pursuits and it's pretty easy to pack away a paper easily.
I can only guess that they were in the middle of 12 hour backgammon championship game that simply couldn't be stopped.
Have you seen any unusual games being played on the Tube (tonsil tennis doesn't count)? Or any games that you think are under utilised in Tube travel?