Two middle-aged men sitting on the train to Bolton are discussing Premier League wages. "£100,000 a week," one of them says slowly, shaking his head. His mate nods: "You'd want to see teams win 9-8 every week for that!" The youth in the Manchester United shirt in the seat opposite doesn't react. This is the world of football as he has always known it – rich beyond compare and a long way from its origins.

Perhaps those three should get off at Salford Crescent station and walk up to the Working Class Movement Library. A small display there, curated by library manager Lynette Cawthra, shines a light on a forgotten part of Man U's history.

The first meeting of the Association Football Players Union in 1907 was chaired by United player Billy Meredith – the "Welsh Wizard", who was one of the first footballing superstars. The union campaigned against FA rules such as the £4 ceiling on wages and the lack of compensation for injured players.

In 1909, the FA withdrew its recognition of the union because it didn't like its authority questioned; players were ordered to resign from the union or have their registration cancelled. Many players did resign, but not all. Heroically, the whole of the Manchester United team refused.

At times, this amazing tale reads like a film script. Charlie Roberts, a local hero and team captain, reputedly heard that they'd all been suspended via a report in the local paper; he and the rest of the team went to the club's offices to have it out with the management, but found only a hapless office boy in charge.

In Behind the Glory, his splendid book about the history of the Professional Footballers' Association, John Harding takes up the story quoting a contemporary account. "Well, something will have to be done," said Sandy Turnbull [ace goalscorer] as he took a picture off the wall and walked off with it under his arm. The rest of the boys followed suit, and looking glasses, hairbrushes and several other things were for sale a few minutes later at a little hostelry at the corner of the ground . . ."

The players, despite being suspended by the club, continued to train locally. One afternoon, a photographer turned up to take a picture. Roberts, never one to miss an opportunity, made a sign that said "The Outcasts FC".

The team now gaze steadily out at you from that photograph in the front room of the Salford library, as reminders of their struggle that leads in a direct line to the bulging wallets of today's players and the structure of football as we know it. As Roberts said: "Try to remember that union is strength, and without it you can do nothing."

But let Meredith have the last, prescient word: "The unfortunate thing is that so many players refuse to take things seriously but are content to live a kind of schoolboy life and to do just what they are told . . . instead of thinking and acting for himself and his class . . . "

The Outcasts display is at the Working Class Movement Library, Salford, until 30 November. wcml.org.uk

Ian McMillan is a poet and broadcaster. ian-mcmillan.co.uk