A train passenger has been fined £462 for mistakenly using his railcard for a cheap journey, as experts say “byzantine” rules are leading to criminal convictions.

The traveller was hauled to court by Northern Rail after incorrectly using the discount card on a 20-minute journey between Prescot and Liverpool Lime Street in January.

It comes after the chief magistrate ordered that 75,000 secretive rail ticket prosecutions should be nullified because train companies had been misusing the law.

The unnamed traveller, whose case was highlighted on Twitter by Bolton News reporter Jack Fifield, was fined £220, alongside a victim surcharge of £88 and costs of £150. The traveller’s train ticket was worth £4.30, court records shared by Mr Fifield said.

It comes after another traveller separately told The Telegraph how Northern had threatened him with court over an alleged £1.90 underpayment.

Sam Williamson, 22, from Glossop in Derbyshire, received a threatening letter from Northern Rail after he took a train from Broadbottom to Manchester on September 5. He was heading to London to sit a driving theory test and had bought an Anytime Day Single.

The Oxford engineering graduate said that he was challenged by a ticket inspector because he was on a train for which a railcard could not be used.

“I tried to offer to pay for a new one,” Mr Williamson said, recalling that the inspector told him: “No, we have to do a travel incident report.”

Mr Williamson was sent a letter from Northern Rail demanding written evidence within a fortnight of October 2. The letter, seen by The Telegraph, said: “Failure to respond to this reasonable request within 14 days of the date of this letter will result in legal action being taken.”

It also states that if it invokes legal proceedings, Northern will ask the court to award it an extra £150 from the traveller “as a contribution to our costs”.

If Mr Williamson is prosecuted, he will have a criminal record. Based on similar previous cases heard by magistrates’ courts, he may have to pay more than £400 in total.

The graduate said: “The fee feels completely and wildly disproportionate in my books, particularly when I don’t think railcards do a good job of clarifying this.”

He said another passenger on his train who was also challenged about not being able to use their railcard had simply walked away from the inspector and got off the train at the next stop.

Mr Williamson said: “If Northern decides to take this to a prosecution, then ultimately what I’ve done by staying on the train and giving all my details and such is the wrong call.”

Rail company prosecutions of passengers have come under intense scrutiny in recent months after the chief magistrate of England and Wales ruled that 75,000 train ticket convictions had been wrongly obtained.

Lawyers for both Northern Rail and Greater Anglia admitted in court that they wrongly prosecuted alleged fare evaders under the controversial Single Justice Procedure (SJP), which sees cases decided in private without the defendant appearing in court.

But train passengers are still being caught out by complicated rules about when railcards can and cannot be used. Customers said train operators’ platforms are allowing the discounts to be applied even to ineligible tickets.

A railcard can be used to get up to half off the price of rail travel but most offer a discount of a third, and cost an average £30 to buy.

The discount cards cannot be used on journeys that cost less than £12 between 4.30am and 10am on weekdays, except in July and August, and on public holidays.

The National Rail Conditions of Travel (NRCoT) state that when a passenger is incorrectly using a time-restricted ticket for that day, they will be charged the difference between what was paid and the cost of the right fare.

The terms and conditions for the 16-25 Railcard, as used by Mr Williamson, say that the discount “has a minimum fare that applies from 04.30 and 10.00 Monday to Friday”, but that this does not apply to “journeys during July and August”.

Gary Rycroft, a consumer lawyer, said that any restrictions on when railcards can be used should be made explicit when consumers buy the discount cards, and then again when they use them to purchase cheaper tickets.

The lawyer said that prosecuting those who don’t understand the “byzantine” rules could be unlawful.

He said: “This saga makes me think more than ever we need real people in ticket offices who will explain this minutia to rail users, and the resources rail companies have should be to enforce those who do not pay at all, rather than those who make a genuine error because the rules are byzantine.”

Jane Hawkes, a consumer champion, added: “Tickets should be greyed out for sale if the railcard is invalid on selected routes or times.”

Under the Consumer Rights Act 2015, terms and conditions are not enforceable if not explained at the time of purchase.

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 96pc – do just that.”

The Department for Transport failed to respond when asked if ministers were monitoring train companies’ prosecutions in light of the SJP scandal.