On-board vehicle, machine

In some systems, passengers are required to re-validate their travel documents on alighting from the vehicle, so that either confirmation is made that the correct fare has been paid or, alternatively, that the fare paid can be adjusted for the trip actually undertaken. This latter feature is common for stored-value products, with the initial fare deducted being that for the full remaining length of the route and any unused travel then subsequently rebated.

To achieve these objectives, the validation equipment is usually placed close to the exit doors on the vehicle. If an incorrect transaction or no transaction at all has taken place, the equipment is designed to issue an audible alarm to alert the driver / crew of the situation. Machine validation is usually faster than manual checking, but it does require a general supervision by the crew and an acceptance of the system by the passengers.