Green’s Dictionary of Slang

the act of mugging, esp. when performed en masse against a ‘captive audience’ by a gang on a bus or, more likely, an underground train.

1989		J. Morton Lowspeak.
1990		J. Healy Streets Above Us (1991) 33: ‘Steaming’, the notorious strategy of mass invasion of a group of travellers by a gang that indiscriminately robs anybody in their way.