John Hannah is partly responsible for the film getting made after the funding collapsed. He coincidentally happened to be in a meeting with Hollywood hotshot Sydney Pollack and casually mentioned this great screenplay he was hoping to make. Pollack was sufficiently interested to read the script and immediately sorted out the funding.
The sliding doors incident that sparks the parallel story was shot at Fulham Broadway station and at Waterloo Station on the Waterloo and City line of London Underground. The blue train is a Waterloo and City line train.
When James and Helen first meet on the tube, James talks about The Beatles. A little later in the film, as Helen and Anna get into the taxi they instruct the driver to take them to 9 Menlove Ave. 251 Menlove Ave in Liverpool was the childhood home of Beatle John Lennon, who felt a great affinity with the number "9".
Peter Howitt is good friends with Brian May, of the band Queen, and gave him a copy of the finished script, asking him to write a song for the film. May penned the song "Another World", but was told a few months into production that they couldn't use it, because another record company was co-financing the production. May kept the song and it became the thematic thread for his solo album "Another World" (1998).
Rowers from London Rowing club, Cygnet, were chosen to be John Hannah's crew mates in the film. The casting director initially went to Imperial College, but when it was noted that they were "all over two meters with flat stomachs" it was decided that lesser rowers than the elite set of internationals would be cast. This is deliberate in order to make Hannah look like the tall athletic one in the boat.