Welcome to the forum.
We probably can't give you a great deal of comfort. Let's start with the really bad news: if the railway are looking to prosecute you, they have up to six months to get in touch with the courts - and even then, the courts can take a few weeks to get in touch with you. But that's the worst it can be: more realistically, it may be two or three months before you hear from the railway.
That's still not good given that you're getting worried about this. So let's look at what's likely to happen. When the railway gets in touch with you, it seems that a lot of the time their first letter will talk about prosecution. But from feedback that we have received here, even at that point a lot of the railway companies will be prepared to let you settle out of court. That will involve you paying the train fare that you say you evaded, and also paying the costs that the train company has incurred investigating the case. So you will be paying them up to a few hundreds of pounds.
If the railways do take you to court, then you'll probably worry about what might happen. You might win at court, although since you have said above that it is 'a fare evasion' I think you are accepting that you dodged a fare - and if you did that, then you don't really have a leg to stand on at court.
If you lose, you will face paying a fine, plus the railway's costs, plus a 'victim surcharge' which is the fare you should have paid. If you look closely at the law, you will see that in theory fare dodging can lead to you going to prison. But in practice it doesn't. So you won't go to prison.
And (again, if you lose at court) you may end up with a criminal record. But that doesn't have to be life changing. If you ever need a DBS (Disclosure and Barring Service) clearance, you will have to admit to your criminal record - but most employers, voluntary organisations and so on will accept that sometimes people do stupid things. They get a lot more worried if you try to hide a criminal record, because that's continuing dishonesty.
So we can't tell you that the railways will be in touch with you soon. But even if they do their worst and throw the book at you, your life will not be ruined. So try to keep calm, learn from this that fare evasion is not a good idea, and otherwise try to get on with your life until the railways get in touch with you.