Thanks. Do you find it’s almost always a case of them sending an open email/letter for you to explain yourself or do they ever just issue a fine within that first letter?
when you talk here about a 'fine' I wonder if you are confusing this with the sum of money seen on warning posters at stations about Penalty Fares (£100 or £50 if paid promptly) - if that is the case that is not what will happen to you now. Such Penalty fares have to be issued on the spot - and more to the point they are not designed for cases of suspected deliberate evasion like short faring - which is why you should not expect that to happen to you know. You have no doubt been suspected of deliberate evasion and the outcome of that is an investigation that can go down 2 route - prosecution in court for fare evasion (if found guilty that results in a court fine (the punishment set by the court for the offence), costs etc and a criminal record)
or an out of court settlement if they agree to that instead of prosecuting you.
The good news is that as explained above, if you follow advice on here it is very likely you can achieve an out of court settlement. You can see many other thread on the forum involving the same railway company as you so have a browse through them to see how things can play out.
The key to this is persuading the railway that you have learned your lesson, won't do this ever again, and understand why what you did was wrong, and the seriousness of it.
If there is something that you can do to show that you have changed your ways then that is something you can do now and mention in your favour (eg buy a season ticket / flexi season ticket) in due course, tho if such a ticket does not work in your favour financially then I don't think you need to buy one - but you do need to buy the correct tickets from now on of course.
Help here will allow you to resolve this in the 'least worst' way I think.