The heavy Ayrshire accent with its uncompromising blunt delivery belies a man whose left-wing instincts are tempered by pragmatism. Mr Knapp, general secretary of the National Union of Rail, Maritime and Transport Workers - formed out of the merger of the National Union of Railwaymen and the National Union of Seamen - sees no advantage in tilting at windmills.
His union's campaign of 24-hour strikes is born of a sincere anger that British Rail may be preparing to break with tradition and make his members compulsorily redundant.
It is also born of a calculation that the Government may still be on the back foot after the outcry over pit closures. Mr Knapp believes there is little public support for the creation of an anorexic railway system fit for privatisation. But there would be little chance of him agreeing to synchronise action with the National Union of Mineworkers unless he smelled ministerial blood.
The complications of his public life are echoed by a complex private existence. His erstwhile image as a staid apparatchik in the chipped- mug bureaucracy of his union has required a degree of revision since he left his wife of 24 years. A relationship with a German divorcee was made public three years ago. He is a thoroughly nice man, but one whose calm can be disturbed by the occasional bout of incandescence when crossed.
The plain man in Mr Knapp emerges in restaurants when he invariably orders steak (well-done) and whisky (with water). Mr Knapp, a man of 52 and tired of being told he looks more like 62, will not be the man to cut a swathe through the creaking and highly centralised administrative machinery of the RMT. However, he can be relied upon to use the bureaucracy to his best advantage.
He told the Independent recently: 'Under no circumstances will I take my members into a dispute unless I'm certain that they will come out of it better-off. I'm not interested in defeats.'
An abiding lesson on pragmatism was learned in 1983 when he took over as general secretary of the old National Union of Railwaymen from the abrasive right-winger Sid Weighell. Mr Knapp pursued with zeal a recommendation by his executive that his members should strike over the introduction of driver-only trains, but was firmly rebuffed in a ballot. That was seen as a humiliation for the new leader and its lesson has been learnt.
His subsequent caution in such matters was demonstrated in 1989 when Mr Knapp ensured he had both his own members and commuters behind him before he led a successful series of 24-hour pay strikes.
Since those days British Rail has promoted managers in its industrial relations department with whom Mr Knapp has a closer relationship. Until now there has been an atmosphere of mutual respect. This dispute will test those amicable relations to their limit.
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