There was the LNER data theft in September 2025,
www.lner.co.uk
: Major UK player cagey on specifics but latest attack follows string blamed on 'third party' suppliers
www.theregister.com
"customer contact details and some information about previous journeys,"
It's plausible that information may have been obtained from that.
For example a season ticket might still be valid?
or you could get regular journeys and make guesses.
They didn't offer much information about exactly what data was stolen.
There was also the TFL leak in 2024
TfL insists it has "kept customers informed throughout this incident and will continue to take all necessary action".
www.bbc.co.uk
I'm not clear that the TFL data theft included journey details, but maybe it did?
The other obvious possibility is a phishing attack.
Sending the "Please confirm your journey details email" and directing people
to a fraudulent site, then using the data in another form.
edit: I've replaced leak with data theft, which I prefer since it implies malicious intent by an outsider rather than simple loss.