The party has already secured key posts in RMT, the biggest rail union, but yesterday's results marks a considerable extension of its influence.
Dave Rix, a relatively unknown hard-left union official from Leeds, polled 4,558 votes compared with 3,357 for Lew Adams, who has led Aslef for four years.
While Mr Adams was a left-wing pragmatist, Mr Rix, 35, belongs to a Marxist party which sees industrial conflict rather than negotiation as the means of winning better working conditions.
Mr Adams is Old Labour and has been a constant thorn in the side of the Government, but is considered right-wing by Mr Rix.
Next in line for takeover by Mr Scargill's party is the RMT where Bob Crow, the deputy general secretary, is a member of the SLP. Jimmy Knapp, 58, the RMT general secretary, will have to decide later this year whether or not to run in a fresh election for the leadership of his union. Four out of the 11-strong rail executive of the union support Mr Scargill's party.
It is understood that if Mr Knapp, also a pragmatic left-winger, decides to step aside, his deputy will probably run and stands a strong chance of victory. With Mr Crow as leader of the RMT and Mr Rix at Aslef, the threat of industrial disruption would loom even larger.
Mr Rix is little known outside his local branch of the union, although he stood in the last general election as a candidate for the SLP. He has worked on the railways since he was 16, joining its Holbeck depot in Leeds as a trainee driver. The Barnsley-born father-of-two has worked alongside Communists in the union and was prominent in supporting the miners during the strike of 1984-5. He regards Mr Scargill as one of the great heroes of the labour movement and himself as a true socialist
The new general secretary, who will start work next January, also knows the union's rule book backwards.
The SLP has exploited dissatisfaction in the union over restructuring schemes signed by Mr Adams with some 30 train-operating companies. While Aslef members welcomed pounds 20,000-pounds 25,000 salaries, they were unhappy about longer driving periods. Fragmentation of the network since privatisation has led to local officials like Mr Rix achieving greater prominence.
Mr Rix has led the left in the union over the last three or four years and with his supporters has made capital out of the decision by the present union leadership to enter a partnership with Richard Branson to form a training company for drivers.
Mr Adams said he was disappointed to lose to an SLP candidate: "Obviously he had a lot of resources behind him for this purpose. I am sad that as a consequence, our union will not now have the same relationship with the Labour government."
Mr Rix indicated yesterday that he would not propose a merger of the two unions, but the chances of such an amalgamation would then be much greater. Moderate union leaders would look with considerable trepidation at the spectre of one powerful union for the rail industry under the leadership of the SLP.
What makes Aslef a potent force is the immediate impact of any industrial action and the virtual impossibility of recruiting staff to replace strikers.Reuse content