An atmospheric railway uses differential air pressure to provide power for propulsion of a railway vehicle. A static power source can transmit motive power to the vehicle in this way, avoiding the necessity of carrying mobile power generating equipment. The air pressure, or partial vacuum (i.e., negative relative pressure) can be conveyed to the vehicle in a continuous pipe, where the vehicle carries a piston running in the tube. Some form of re-sealable slot is required to enable the piston to be attached to the vehicle. Alternatively the entire vehicle may act as the piston in a large tube or be coupled electromagnetically to the piston. Several variants of the principle were proposed in the early 19th century, and a number of practical forms were implemented, but all were overcome by unforeseen disadvantages and discontinued within a few years. A modern proprietary system has been developed and is in use for short-distance applications. Porto Alegre Metro airport connection in Porto Alegre, Brazil, is one of them. History In the early days of railways, single vehicles or groups were propelled by human power, or by horses. As mechanical power came to be understood, locomotive engines were developed; the iron horse. These had serious limitations, in particular being much heavier than the wagons in use, they broke the rails; and adhesion at the iron-to-iron wheel-rail interface was a limitation, for example in trials on the Kilmarnock and Troon Railway. Many engineers turned their attention to transmitting power from a static power source, a stationary engine, to a moving train. Such an engine could be more robust and with more available space, potentially more powerful. The solution to transmitting the power, before the days of practical electricity, was the use of either a cable system or air pressure. Medhurst In 1799, George Medhurst of London discussed the idea of moving goods pneumatically through cast iron pipes, and in 1812, he proposed blowing passenger carriages through a tunnel. Medhurst proposed two alternative systems: either the vehicle itself was the piston, or the tube was relatively small with a separate piston. He never patented his ideas and they were not taken further by him. 19th century Vallance In 1824, a man called Vallance took out a patent and built a short demonstration line; his system consisted of a diameter cast iron tube with rails cast in to the lower part; the vehicle was the full