They spent a lot on advertising to begin with, specifically advertising that broke the guidelines, but was only taken down after everyone had heard the name and the idea that it was cheaper to use Trainline: https://www.theguardian.com/media/2009/may/20/asa-trainline-ad https://www.railforums.co.uk/threads/misleading-advertising-by-trainline.104250/

They eventually started getting commissions from train operating companies (TOCs) for selling tickets, which I believe the TOCs in turn used as justification for raising ticket prices above inflation. I used to work for a TOC that at the time refused to work with Trainline, because it meant having to install special ticket machines at loss only to make more loss on ticket revenue, but according to a mate of mine who still works there, they now accept the tickets because it's such a headache for staff when people come in having spent money for an invalid ticket.

So it's just shit for everyone except Trainline, and quite a stark example of how unsuited for public infrastructure a free market model is. Imagine if some company came along and said you could pay your gas bill through them for cheaper and it just wasn't true but everyone believes it so gas is just dearer now. Fucking bananas.