Being issued with a Penalty Fare means that this incident is now dealt with (and you might think they were lenient to issue a PF and not threaten you with prosecution for something blatant like buying a child ticket if you're not a child). However, they obviously have your details now; and if you buy tickets online (rather than from a station machine or ticket office) your previous ticket purchases are of course all traceable (irrespective of where you bought them online). So if you've done it before, and they look at your history (as they might well do), then they can still chase you up over previous invalid tickets. In which case it won't be just a Penalty Fare...
Only you know what they might find; but they know that it would be a coincidence if you were found with a deliberately wrong ticket and it was the only time you'd ever done it - since people don't necessarily get caught the first time. So do be prepared to hear from them again if there's a dodgy ticket-buying record that they might find.... And meanwhile, now you're on their radar, be really careful in future to always have a valid ticket before you step on a train.
