Train companies have admitted overcharging thousands of passengers caught fare dodging.
Industry body the Rail Delivery Group (RDG) told the PA news agency that eight train companies punished ticketless travellers too harshly by not implementing a rule change made in April last year.
The new legislation reduced the correct punishment for some fare dodgers depending on the time of their journey.
But the RDG issued guidance to rail firms which failed to correctly set out the updated rules, resulting in them being ignored.
Up to 10,000 passengers could have been affected by the mistake, which was only discovered last month and led to tens of thousands of pounds being unfairly demanded by ticket inspectors.
The RDG said it has conducted an investigation and will ensure train staff are properly trained.
A passenger watchdog demanded that train operators must do “everything possible” to prevent a repeat of the error.
Around half of people affected will be issued refunds for the amount they were overcharged, but in many cases the train companies do not have fare dodgers’ full contact details.
Posters will be displayed at stations in the coming weeks to advise anyone who thinks they may be owed money to get in touch with the relevant operator.
The train companies which overcharged passengers are Chiltern Railways, Govia Thameslink Railway, Great Western Railway, Greater Anglia, Northern, Southeastern, South Western Railway and London Northwestern.
Around 1,500 Southeastern passengers were affected by an average of £8.
London Northwestern Railway estimates it overcharged 2,700 people by a total of £12,000 – which works out at £4.44 per person.
Great Western Railway said it will post cheques to those passengers it has details for, with the average refund being £6.
An RDG spokesman said: “When people haven’t paid to travel, it’s important for train companies to take a firm but fair approach because fare dodging denies the railway around £200 million a year which could otherwise be invested to improve services for all passengers.
“People who have been charged a penalty fare shouldn’t be overcharged, though.
“We have investigated this issue and will ensure that staff have the right advice and people affected are reimbursed quickly and easily.”
David Sidebottom, director of watchdog Transport Focus, said: “Passengers will want urgent answers as to why this error occurred and assurances from the rail industry that it is being investigated and fixed immediately.
“Train companies need to do everything possible to demonstrate that they have a clear plan to get things back on track, provide train staff with the correct advice to avoid this happening again and make sure passengers aren’t out of pocket.
“Only then will people feel confident that they are being treated fairly.”
On routes where penalty fares apply, passengers must buy a ticket before boarding a train at stations where there is an open ticket office or a functioning ticket machine.
If a ticket inspector catches anyone who avoids doing this, they can issue them with a penalty fare notice.
These are also handed out to passengers unable to produce a railcard after buying a discounted ticket, travelling in first class with a standard ticket or staying on a train beyond the destination paid for.
Penalty fares are £20 or twice the appropriate single fare, whichever is higher.
The single fare was previously based on the price of a ticket valid at any time of day.
But since April last year the price of off-peak fares should be applied if the tickets were available for the journey taken.