Train passengers who forget their railcards will be able to claim a refund if they're forced to pay a higher fare while travelling on discounted journeys but you'll only be able to claim back cash once a year.
Previously anyone who'd bought a ticket using the railcard discount, which usually gives you about a third off fares, had to be carrying the card on their journey or else face paying more or being given a penalty fare.
The penalty would be £20 or twice the single fare, whichever is greater.
From today, however, all train companies have agreed that if a passenger is asked to pay extra when they forget their card, they will be guaranteed a refund the first time it happens each calendar year (provided they can prove they held a valid railcard at the time of travel).
There are currently six UK railcards on offer, including those that cater for passengers aged under 26, over-60s, and for those travelling with a companion. See our Cheap Trains guide for more information and our Railcard deals page for offers.
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How do I claim the refund?
There's no claims form to do this so the best thing to do would be to contact the train company running the service, via email, phone or letter.
Provide as much information as possible including any receipts for the extra fare you paid, copies of your railcard and original tickets, and any other information such as the time of the train's departure and the date you were travelling on.
Our Train Delays guide has contact details for each train company.
Why are the rules changing?
The changes are part of an action plan which has been agreed by train companies, following a campaign by Which? and industry watchdog Transport Focus, which was supported by MoneySavingExpert.com.
The action plan, unveiled in December last year, includes a raft of measures to ensure passengers can find cheaper tickets. Further changes will include more companies offering advance tickets to buy on the day you travel and removing jargon such as 'London terminals' from ticket machines.
Jacqueline Starr, managing director of customer experience at the Rail Delivery Group, which represents train companies, said: "Customers make honest mistakes sometimes we think railcard holders should have one chance a year to be refunded if they've had to pay extra for forgetting theirs.
"We're planning digital railcards too that people can keep on smartphones and other devices if they prefer, making them harder to forget and easier to replace if lost or stolen."
Anthony Smith, chief executive of Transport Focus, added: "It is good news that passengers will no longer suffer a heavy penalty for making the mistake of not having their railcards with them.
"This is an example of how our complaints postbag has driven change to benefit passengers. For a number of years, we have seen how passengers can be unfairly and harshly treated when making an honest mistake."