Ticketless train travel that works by tracking passengers' locations is being trialled in the East Midlands.
The location-based technology will be tested by East Midlands Railway (EMR) from Monday, by up to 1,000 people travelling through Derby, Leicester, Nottingham and the stations in between.
The app promises to automatically charge these passengers "the best fare at the end of the day".
Further 1,000-person trials will take place on three services run by rail operator Northern in Yorkshire from the end of September, the Department for Transport said.