Jimmy Knapp died last year

The union is only the sixteenth biggest in Britain, but in recent weeks it has been the focus of industrial action in a variety of regions of the UK, including those that disrupted South West Trains services.

Mr Crow beat Phil Bialyk and Raymond Spry-Shute to win his new job, leaving both the main rail unions with left-wing general secretaries.

The train drivers' union Aslef is led by Mick Rix, a former member of the Arthur Scargill's Socialist Labour Party who has since rejoined Labour.

Recent weeks have seen industrial action at a number of train companies including South West Trains and ScotRail and there have been fears that the disputes could merge into a national strike.

Pay dispute

While stoppages at South West Trains have been suspended pending talks with the company, the RMT plans walkouts at Arriva on 1 and 2 March in a dispute over guards' pay.

The rail unions argue that the fragmentation of the industry has made reaching agreements much more difficult.

They want a return to national pay bargaining.

While the Left is in the ascendant in rail union elections, there are some signs that rank and file members may be losing their enthusiasm for strike action.

South West Trains believes last week's decision to suspend strike action followed pressure from members unwilling to lose more days' pay.

Mr Crow polled 12,051 votes compared with 4,512 for Mr Bialyk, the union's Bristol-based regional official and 1,997 for Mr Spry-Shute, RMT education and training officer.

London Underground managing director Paul Godier said: "We look forward to the new general secretary rising to the challenge of modernising industrial relations in the railway industry in the best interests of both customers and staff."