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https://en.wikipedia.org/wiki/Stowaway
A stowaway is a person who secretly boards a vehicle, such as an aircraft, bus, ship, cargo truck or train, to travel without paying and without being detected.
Stowaways may risk being fined or imprisoned, since it is illegal in most jurisdictions to embark on aircraft, boats or trains as stowaways. Airports, sea ports and train stations are typically marked as "no trespassing" or "private property" zones to anyone but customers and employees. Seaports, train stations, and airports often attempt further security by designating restricted areas with signs saying "authorized personnel Only".
Since the September 11 attacks, it has become more difficult to be a stowaway on board transportation arriving to or departing from the United States. Airport security has dramatically increased, and among the new security measures is watching over the fences from which stowaways usually gain entrance to an airport's runway.