Labour says its policy would be enforceable in law
Labour is going to include in its manifesto a commitment to force rail companies to offer passengers the cheapest fare, the BBC has learnt.
Rail passenger numbers are at the highest level since World War II, but consumers often complain that tickets are mis-sold.
The Tories say they would revamp the rail franchising system to give a better service.
The Lib Dems have pledged that fares would rise by less than inflation.
Labour says that if it wins the election it will give passengers an enforceable right to the cheapest fare, the BBC understands.
Consumers would be able to get a refund if they were mis-sold a ticket, with disputes going to the regulator.
When the consumer group Which? investigated rail fares, it found that the cheapest ticket was offered on fewer than half of occasions.
Researchers were not told about cheaper train firms, off-peak fares or season ticket savings.
However, the train companies said that mystery shopper investigations see the right ticket sold 99% of the time.