Isaac AsheBBC News, Derby
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.
The digital ticketing uses global positioning system (GPS) tracking on users' mobile phones to automatically charge passengers the best overall fare at the end of the day, including on CrossCountry as well as EMR.
If multiple journeys are made, the government said it would calculate if a season ticket would have been cheaper.
For ticket inspections and to pass through barriers, the app will generate a bar code to scan.
Passengers can sign up to the trial, called Digital Pay As You Go, through EMR's website.
The technology has already been tested widely in Switzerland, Denmark and Scotland but this marks the first time the trials have been brought to England's rail network.
EMR head of commercial strategy and business planning Oli Cox said more than 500 people had already signed up.
He said "complex" fares were sometimes "a real barrier" for passengers and added: "This trial removes that uncertainty, making it easy to simply tap in and out on your phone, safe in the knowledge you're always getting the best-value fare on the day."
EMR added the trial would not affect the decision to install ticket barriers limiting access to platforms from the footbridge through Nottingham railway station.
Trials will begin running on Northern services between Harrogate, Leeds, Sheffield, Doncaster and Barnsley at the end of the month.
The EMR and Northern trials will run for nine months and have been given £1m of government funding as part of its Plan for Change.
Rail minister Lord Peter Hendy said testing ticketless travel should save passengers both time and money.
He said: "The railway ticketing system is far too complicated and long overdue an upgrade to bring it into the 21st Century."
The trial has received a mixed response from passengers, with some saying they would rather see rail bosses prioritising things like upgrading air-conditioning and improving carriage cleanliness.