The 1996 black comedy about drug addicts living in Edinburgh became one of the most revered films of the decade and made a star of Ewan MCGregor.
Its director Danny Boyle included a number of classic rock songs on its soundtrack, including Iggy Pop's Lust for Life and Lou Reed's Perfect Day, as well as original music from indie rockers Primal Scream.
When approached for a contribution, Noel refused as he thought it was about hobbyists who document the identity of trains they see travelling on rail lines.
According to editors at Scotland's Edinburgh Evening News, the film's producer Andrew MACdonald and costume designer Rachel Fleming revealed the strange story at an Edinburgh event celebrating the film's 20th anniversary.
"Danny is from near Manchester and he was very keen to have Noel Gallagher do something but there was a reason why he didn't do it," Andrew said. "He came to the launch party in Cannes (France), but I don't know why he didn't do a piece of music."
Rachel explained that she had met the Supersonic musician at a recent event and he had told her why he turned down the chance to appear on the soundtrack.
She said, "I met Noel at a thing the other week and he said to me: 'I would have done something, but honestly I thought it was about trainspotters. I didn't know.' That's what he actually said."
Danny and the film's stars Ewan, Robert Carlyle and Jonny Lee Miller are set to return in a long-awaited sequel, T2: Trainspotting, in January (17).