Why not? Isn't the passenger in violation of the railway bylaws, for which "I accidentally got on the wrong train" is not a valid defence?
Obviously I'm not arguing that the passenger should be fined in these circumstances, but isn't it at least theoretically possible?
Good question.
You are correct to say that it is "at least theoretically possible" for the front-line member of staff who comes across such an incident to refuse travel and/or to take a note of the evidence of the facts to be presented for consideration by an appropriate Investigator for a Prosecution, and then at the end of their turn or shift, to hand them in.
And it is correct to say that it is "at least theoretically possible" for the Company's revenue protection team to consider the facts and decide to pursue a Prosecution against that passenger.
And it is correct to say that it is "at least theoretically possible" for them to instruct their Prosecutor and to apply a form of the 'full code test' to confirm that a prosecution would be both in the public interest and that the evidence id adequate to make a conviction likely.
And it is correct to say that it is "at least theoretically possible" for the team and the Prosecutor to both make the irrational decision that it is appropriate to prosecute.
And it is correct to say that it is "at least theoretically possible" for the Court to list the Prosecution for a hearing.
And it is correct to say that it is "at least theoretically possible" for the Court to find that the facts as stated lead them 'beyond all reasonable doubt' to find the passenger Guilty.
And it is correct to say that it is "at least theoretically possible" for pigs to fly.
I'm not expecting either of those outcomes ever to be reported, even the committed anti-rail members of this forum would appear to be struggling to do so if asked to provide real examples of actual convictions in such circumstances. I believe that Pink Floyd provided some evidence of flying pigs over the south bank of the Thames.