An investigation is under way to try to find out how much money transport companies are earning from people who unwittingly forget to check out of trains, buses and trams while using public transport smart cards.

The probe has been set up by tourism group ANWB and rail users’ lobby Rover.

They expect significant amounts of money are involved, Nos television reports. The results of the investigation are due in the autumn and will be used to try to work out ways to solve the problem.