Revenue protection inspector

Revenue protection inspector (RPI) or revenue protection officer (RPO) is the job title given to staff who patrol different forms of public transport issuing penalty fares to passengers who travel without a valid ticket or without the correct ticket (e.g. travelling where the ticket is not valid or a commuter using a concession ticket without possessing appropriate evidence of entitlement to concession).

Source: Wikipedia — Revenue protection inspector (CC BY-SA 4.0)