Yes, as
@Fawkes Cat says if they agree to settle with you it won't be a 'fine' but you will, going from other cases on here be asked to pay (hopefully, as that will avoid court):
- a sum for the cost of their admin of dealing with you / this case. Maybe up to £200 perhaps
- the cost of the full train fare from your home to destination - I would expect at the Anytime / peak travel time costs to be used (not off peak fare even if your shifts were off peak for example)
- that fare multiplied by the number of days you have (already at interview or in your response later) indicated you have traveled
- they may assume you work 5 days a week and have thus evaded the correct fare for 5 days a week, you may need to be able to prove / show / state clearly when you get the chance to send your written response that you do not go in 5 days a week
BUT - you will not be a in strong position to 'negotiate' over the sum since they have evidence that would permit them to take you to court where you will be almost certainly found guilty (as you have admitted what you did) and the longer term consequences of that would be, I think, for you something you wish to avoid, and would be more costly overall I suspect.
So best to start saving as much as you can so if they ask you for a settlement offer you are able to pay it promptly and in full as advised above.
This is also why preparing your short but sincere apology and remorse letter is maybe a useful thing you can do now whilst you await them contacting you about this, since that will be your reply, essentially.