Keir Moffatt, 26, kicked himself for failing to ask out the “just gorgeous” girl he met for mere moments. So he set up a Facebook page to try to track her down.
In just over 10 days, the romantic odyssey had gripped well-wishers from America, Australia and Malaysia.
Moffatt, 26, who lives in Bristol, was travelling home from Cardiff at 7pm on May 6, when he saw a girl with a blue hair band, sparking the search.
On the website he wrote: “On the train back to Bristol from Cardiff last week, a gorgeous girl in my carriage really caught my eye.
“We barely spoke but, when she left, I regretted not speaking to her more – and asking her out.
“I set up this Facebook group to see if I could find the Mystery Train Girl and get a second chance to ask her out.”
By the following evening more than 1,000 people had joined the group, which soon swelled to 2,000, and then 10,000 members. Moffatt decided to put the interest to use, setting up a JustGiving web page, asking his followers to donate £1 to Marie Curie Cancer Care.
So far, he has collected more than £830 (€970), and attracted 13,324 Facebook followers, despite calling off the search for fear of inviting unwanted attention onto the girl.
Moffatt said: “I figured it was possible she did not want to be found (pretty understandable!) and it was best to respect that, so I called off the search.”