patent, which became successful. The patent application cited the increase in air travel, and "baggage handling [having] become perhaps the single biggest difficulty encountered by an air passenger", as background of the invention. Sadow's four-wheeled suitcases, pulled using a loose strap, were later surpassed in popularity by suitcases that feature two wheels and are pulled in an upright position using a long handle. These were invented in 1987 by US pilot Robert Plath, and initially sold to crew members. Plath later commercialized them, after travelers became interested after seeing them in use by crew members, and founded the Travelpro company, which marketing the suitcases under the trademark "Rollaboard". The terms rollaboard and roll-aboard are used generically, however. While initially designed for carry-on use (to navigate through a large terminal), as implied by the analogous name, similar designs are also used for checked baggage. More recently, four-wheeled luggage with casters has become popular, notably since their use by Samsonite in the 2004 version of their signature Silhouette line. These are otherwise similar in design to two-wheel roll-aboards, with a vertical orientation and a retracting handle, but are designed to be pushed beside or in front of the traveler, rather than pulled behind them. These are often referred to as "spinner" luggage, since they can spin about their vertical axis. Sadow attributes the late invention of luggage on wheels to a "macho thing" where "men would not accept suitcases with wheels". Others attribute the late invention to "the abundance of luggage porters with carts in the 1960s, the ease of curbside drop-offs at much smaller airports and the heavy iron casters then available." Hand/carry-on Passengers are allowed to carry a limited number of smaller bags with them in the vehicle, these are known as hand luggage (more commonly referred to as carry-on in North America), and contain valuables and items needed during the journey. There is normally storage space provided for hand luggage, either under seating, or in overhead lockers. Trains often have luggage racks at the ends of the carriage near the doors, or above the seats if there are compartments. On aircraft, the size and weight of hand luggage is regulated, along with the number of bags. Some airlines charge for carry-on over a certain number. Smart Smart luggage is baggage that has a built-in or a removable battery within. It often includes features designed to help with travel, including