What does ‘travel without a valid ticket’ mean?

If you are unable to present a ticket that is valid for all of the route you are travelling, you will be given a ‘travel without a valid ticket’ document. If you travel without a valid ticket, you will pay a surcharge in addition to the fare. 

Whether you didn’t have enough to time to buy a ticket, or you checked in too late, or if you were no longer able to find the ticket, it makes no difference. Furthermore, your personal details will be electronically recorded and kept for at least two years. You will be charged both the cost of the ticket and the supplement.