Steaming is a slang

Slang

Slang is the use of informal words and expressions that are not considered standard in the speaker's language or dialect but are considered more acceptable when used socially. Slang is often to be found in areas of the lexicon that refer to things considered taboo...

 term used by the British commonly for robbery performed on train or bus passengers by a gang or large group and often involving some level of violence. It is often used interchangeably with mugging

Robbery

Robbery is the crime of taking or attempting to take something of value by force or threat of force or by putting the victim in fear. At common law, robbery is defined as taking the property of another, with the intent to permanently deprive the person of that property, by means of force or fear....

, referring to a specific subcategory of that crime. Several cases have been reported on the London Underground

London Underground

The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...

and other city public transport systems although not exclusively localised to them. It may also be applied to other theft such as shoplifting, involving a large group employing distraction tactics, intimidation or actual violence against staff. In general, the term refers to the operation in a large group and the use of intimidation and violence to commit theft.