Number of unlinked passenger trips taken on public transportation. Unlinked passenger trips is the number of times passengers board public transportation vehicles. Passengers are counted each time they board vehicles no matter how many vehicles they use to travel from their origin to their destination and regardless of wether they pay a fare, use a pass or transfer, ride for free, or pay in some other way. Public transit systems include buses, subways, light rail, commuter rail, trolleys and ferries. Based on: U.S. Department of Transportation.