When do I need a Permit to Travel?
In areas where penalty fares may be issued, if you cannot buy a ticket at the station a permit to travel should be purchased from the machine, where provided. You are expected to insert coins up to the value of the fare or as many coins as you have and this permit is redeemed en route as credit toward the price of the ticket which you could not buy. Without a permit to travel you may be issued with a penalty fare.
Fares and ticketing