CHRISTMAS in London means more than cotton-wool snow in the shop windows, patient Father Christmases, As the days begin to hurry to their Public 391 Seasonal and the tall tree in Trafalgar Square. climax a gentle fever descends on Londoners. They share their umbrellas with strangers; they become erratic but splendid shoppers for mistletoe. London Transport shares this sea- sonal emotion, although Christmas, when added shoppers and sightseers are to its daily millions of passengers, adds considerably to its problems.