A passenger travels on the London underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers travel on the London train and underground system during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





A passenger travels on the London underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers travel on the London underground system during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





A passenger rests as he travels on the London train and underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers travel on the London train and underground system during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





A man rests at Liverpool Street station during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





Young men sing outside Victoria station in London during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers travel on the London train and underground system during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





A man rests outside Liverpool Street station during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





A passenger travels on the London underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





A passenger travels on the London underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers travel on the London train and underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers gather outside Liverpool Street station in London during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





A passenger travels on the London underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers travel on the London train and underground system during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





A passenger east fast food as he prepares to travel on the London underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers gather outside Liverpool Street station in London during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





Passengers travel on the London train and underground system during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





Passengers travel on the London train and underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





A passenger travels on the London underground system during the Christmas party season, December 17, 2015. (Photo by Paul Hackett/Reuters)





Passengers gather outside Liverpool Street station in London during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)





A man is attended to by friends after falling at Liverpool Street station during the Christmas party season, December 18, 2015. (Photo by Paul Hackett/Reuters)