Rail ticket machines cause so much confusion that one-fifth of passengers who use them buy the wrong ticket, according to the rail regulator.
While 7% of people underpay and could be fined, 13% are paying too much.
The Office of Rail and Road (ORR) wants train companies to refund passengers who accidently buy tickets which are too expensive for their journey.
Train operators say plans to simplify fares will help them to give clearer information on ticket machines.
The ORR employed mystery shoppers to buy rail tickets from machines.
- Nearly two-thirds (65%) could not see any information on the machines about the type of tickets which could or could not be bought
- More than half (57%) said the machines did not explain the times when peak and off-peak tickets were valid
- And nearly one-third (32%) said there was no information on the machines about ticket restrictions
"Despite investment in new technology and the removal of jargon from ticket machines, our new research shows passengers may be paying more for their journey than necessary," said John Larkinson, ORR director of railway markets and economics.
"We are calling on train companies to commit to refund anyone who finds that they could have bought a cheaper ticket for the same journey," he added.
Rail operators C2C and Scotrail already have a price guarantee in place for when passengers overpay.
Earlier this month, the Rail Delivery Group (RDG), which represents train operators, announced a trial scheme to simplify fares.
It said the 16 million fares on offer were "baffling" for passengers.
As part of the trial, which begins in May, ticket machines on stations will also be updated, to make sure they show the cheapest fares available.
"To quickly benefit passengers, the Rail Delivery Group must set out what improvements to ticket machines will be made in the short term," said Mr Larkinson.
Responding to the ORR study a spokesman for RDG said: "Complex, decades-old government rail fare regulations make it more difficult for train companies to offer the right, simple options on ticket machines.
"Simplifying the number and types of fares in the system will let train companies provide customers with clear information and help them to make better informed choices," they added.