Mr Williamson bought the cheapest anytime single ticket he could find using his 16-25 railcard for 10:29 BST on Thursday.
He admitted it was his mistake, but said it was not made clear when he bought the ticket on the Northern app.
Mr Williamson said he could not believe an “innocent mistake over a confusing and opaque rule that only saved me £1.90, will lead to a punishment of hundreds of pounds and a criminal record”.
In a post on X, external seen by millions of users, Mr Williamson called on Northern to make it clearer that “an anytime ticket is not any time with a railcard”.
He said “ambiguous” railcard restrictions should be more clearly publicised, and said he should have been given the chance to resolve the issue with the conductor.
“Why would anyone get the train if this is how they will treat you.”
A Northern spokesman said: “As with all train operators across the UK, everyone has a duty to buy a valid ticket or obtain a ‘promise to pay’ voucher before they board the train and be able to present it to the conductor or revenue protection officer during a ticket inspection.
“The overwhelming majority of our customers – upwards of 96% – do just that.”
National Rail, which operates the railcard system, has been contacted for comment.
