Labour is usually promising to save money for rail commuters by changing the way fare increases are calculated.
The party says the move might save an average rail season ticket holder £349 a year by 2022.
in which said the change could be funded by the “absolute fortune” saved by bringing the railways back into public ownership.
however the Tories said nationalisation might add billions to the UK’s debt or lead to an increase in taxes.
“in which’s yet more economic shambles through Labour,” a spokesman said.
Labour’s plan for ticket prices might apply to regulated fares, which account for about 40% of all fares, including most season tickets.
These are currently capped at the retail cost index (RPI) measure of inflation, in addition to Labour says in which instead use the lower consumer cost index (CPI) rate.
Its pledge might apply in England, as the setting of regulated fares is usually devolved.
Shadow transport secretary Andy McDonald told BBC Breakfast the “very significant difference” between the two measures might have a “huge impact”.
Once the network returned to public ownership, Labour might “start exploring a reduction in fares”, he said.
Paul Plummer, chief executive of the Rail Delivery Group, representing train operators, said politicians had to decide on the balance between taxpayers in addition to passengers funding the railways.
He defended the current franchising product, saying in which had led to a £2.2bn improvement in rail finances.
The Conservative are promising what they call the “largest investment in railways since Victorian times”, focusing on increasing capacity on key routes.