They film their creations and sell the footage to specialist distributors for internet music magazines, or the city's avant garde pubs. Split-second timing is the key. One member of the "commando" - which can number up to 20 - travels on the train. At a given moment and place, he pulls the emergency cord and the train stops. The artists leap into action and - crucial to the financial success of the operation- film themselves in front of their creation before escaping.