A monorail is a railway in which the track consists of a single rail. The term is also used to describe the beam of the system, or the trains traveling on such a beam or track. The term originates from joining "mono" (one) and "rail" (rail), from 1897, possibly from German engineer Eugen Langen, who called an elevated railway system with wagons suspended the Eugen Langen One-railed Suspension Tramway (Einschieniges Hängebahnsystem Eugen Langen).
Colloquially, the term "monorail" is often used to describe any form of elevated rail or people mover. More accurately, the term refers to the style of track,[note 1] not its elevation, with 'Mono' meaning 'one,'  and 'Rail' indicating the type of track structure utilized.
Monorails have found applications in airport transfer and medium capacity metros. To differentiate monorails from other transport modes, the Monorail Society defines a monorail as a "single rail serving as a track for passenger or freight vehicles. In most cases rail is elevated, but monorails can also run at grade, below grade or in subway tunnels. Vehicles either are suspended from or straddle a narrow guide way. Monorail vehicles are wider than the guide way that supports them."
Monorail vehicles often appear similar to light rail vehicles, and can be manned or unmanned. They can be individual rigid vehicles, articulated single units, or multiple units coupled into trains. Like other advanced rapid transit systems, monorails can be driven by linear induction motors; like conventional railways, vehicle bodies can be connected to the beam via bogies, allowing curves to be negotiated.
Unlike some trams and light rail systems, modern monorails are always separated from other traffic and pedestrians. They are both guided and supported via interaction with the same single beam, in contrast to other guided systems like rubber-tyred metros, the Sapporo Municipal Subway; or guided buses or trams, such as Translohr. Monorails do not use pantographs.
From the passenger's perspective, monorails can have some advantages over trains, buses, and automobiles. As with other grade-separated transit systems, monorails avoid red lights, intersection turns, and traffic jams. Surface-level trains, buses, automobiles, and pedestrians can collide each one with the other, while vehicles on dedicated, grade-separated rights-of-way such as monorails can collide only with other vehicles on the same system, with much fewer opportunities for collision. As with other elevated transit systems, monorail passengers enjoy sunlight and views and by watching for familiar landmarks, they can know better when to get off to reach their destinations. As with other systems, expensive and noisy ventilation systems are not necessary if the cars have traditional windows that can be opened by passengers. (This also eliminates the weight and bulk of ventilation systems.) Monorails can be quieter than diesel buses and trains. They obtain electricity from the track structure, eliminating costly and, to many people, unsightly overhead power lines and poles. Compared to the elevated train systems of New York, Chicago and elsewhere, a monorail beamway casts a narrow shadow. (See 'Chicago "L"' )
Under the Monorail Society's beam-width criterion, some, but not all, maglev systems are considered monorails, such as the Transrapid and Linimo. Maglevs differ from other monorails in that they do not (normally) physically contact the beam.
The first monorail prototype was made in Russia in 1820 by Ivan Elmanov. Attempts at creating monorail alternatives to conventional railways have been made since the early part of the 19th century. 
Around 1879 a "one-rail" system was proposed independently by Haddon and by Stringfellow, which used an inverted "V" rail (and thus shaped like "?" in cross-section). It was intended for military use, but was also seen to have civilian use as a "cheap railway." 
Early designs used a double-flanged single metal rail alternative to the double rail of conventional railways, both guiding and supporting the monorail car. A surviving suspended version is the oldest still in service system: the Wuppertal monorail in Germany. Also in the early 1900s, Gyro monorails with cars gyroscopically balanced on top of a single rail were tested, but never developed beyond the prototype stage. The Ewing System, used in the Patiala State Monorail Trainways in Punjab, India, relies on a hybrid model with a load-bearing single rail and an external wheel for balance. One of the first systems put into practical use was that of French engineer Charles Larigue, who built a line between Ballybunion and Listowel in Ireland, opened in 1888 and closed in 1924 (due to damage from Ireland's Civil War). It uses a load-bearing single rail and two lower, external rails for balance, the three carried on triangular supports.
In 1910, the Brennan gyroscopic monorail was considered for use to a coal mine in Alaska. In June 1920, the French Patent Office published FR 503782, by Henri Coanda, on a 'Transporteur Aérien' -Air Carrier.
The first half of the 20th century saw many further proposed designs that either never left the drawing board or remained short-lived prototypes. One of the first monorails planned in the United States was in New York City in the early 1930s, scrubbed for an elevated train system.
In the latter half of the 20th century, monorails had settled on using larger beam- or girder-based track, with vehicles supported by one set of wheels and guided by another. In the 1950s, a 40% scale prototype of a system designed for speed of 200 mph (320 km/h) on straight stretches and 90 mph (140 km/h) on curves was built in Germany. There were designs with vehicles supported, suspended or cantilevered from the beams. In the 1950s the ALWEG straddle design emerged, followed by an updated suspended type, the SAFEGE system. Versions of ALWEG's technology are used by the two largest monorail manufacturers, Hitachi Monorail and Bombardier.
In 1956, the first monorail to operate in the US began test operations in Houston, Texas. Later during this period, monorails were installed including at Disneyland in California,Walt Disney World in Florida, Seattle, and Japan. Monorails were promoted as futuristic technology with exhibition installations and amusement park purchases, as seen by the legacy systems in use today. However, monorails gained little foothold compared to conventional transport systems. In March 1972, Alejandro Goicoechea-Omar had patent DE1755198 published, on a 'Vertebrate Train', build as experimental track in Las Palmas de Gran Canaria, Spain.
From 1950 to 1980, the monorail concept may have suffered, as with all public transport systems, from competition with the automobile. Monorails in particular may have suffered from the reluctance of public transit authorities to invest in the perceived high cost of un-proven technology when faced with cheaper mature alternatives. There were also many competing monorail technologies, splitting their case further. One notable example of a public monorail is the AMF Monorail that was used as transportation around the 1964-1965 World's Fair.
This high-cost perception was challenged most notably in 1963 when the ALWEG consortium proposed to finance the construction of a major system in Los Angeles in return for the right of operation. This was turned down by the city authorities in favour of no system at all, and the later subway system has faced criticism[by whom?] as it has yet to reach the scale of the proposed monorail.
Several monorails[example needed] initially conceived as transport systems survive on revenues generated from tourism, benefiting from the unique views offered from the largely elevated installations.
From the 1980s, most monorail mass transit systems are in Japan, with a few exceptions. Tokyo Monorail, today one of the world's busiest, averages 127,000 passengers per day and has served over 1.5 billion passengers since 1964. Monorails have seen continuing use in niche shuttle markets and amusement parks.
Modern mass transit monorail systems use developments of the ALWEG beam and tire approach, with only two suspended types in large use. Monorail configurations have also been adopted by maglev trains. Since the 2000s, with the rise of traffic congestion and urbanization, there has been a resurgence of interest in the technology. Many cities today are seeing monorails as a possible mass transit solution.Chongqing Rail Transit in China has adopted a unique ALWEG-based design with rolling stock that is much wider than most monorails, with capacity comparable to heavy rail. This is because Chongqing is criss-crossed by numerous hills, mountains and rivers, therefore tunneling is not feasible except in some cases (for example, lines 1 and 6) due to the extreme depth involved. Today it is the largest and busiest monorail system in the world. São Paulo, Brazil is building a Bombardier Innovia Monorail system as part of its public transportation network. The 14.9 mile guideway will have 17 stations, 54 monorail trains and a passenger capacity of 40,000 commuters per hour in each direction. Another city installing a Bombardier Innovia Monorail system in an urban centre is Riyadh, Saudi Arabia, for its new King Abdullah Financial District.India is proposing monorails in several cities for mass rapid transit; Mumbai Monorail would be the first one. In December 2014, the government of Malta proposed a monorail system to the European Commission as an infrastructure project to benefit from EU funding. The network would be 76 km (47 mi). However, as of 2016, the project is still in the proposal stages with no funding source secured. China, already home to the world's largest and busiest monorail system has a number of mass transit monorails under construction in several of cities. A Bombardier Innovia Monorail based system is under construction in Wuhu and several "Cloudrail" systems developed by BYD under construction a number of cities such as Bengbu, Anyang and Shantou.
Modern monorails depend on a large solid beam as the vehicles' running surface. There are a number of competing designs divided into two broad classes, straddle-beam and suspended monorails.
The most common type is the straddle-beam, in which the train straddles a steel or reinforced concrete beam 2 to 3 feet (0.6 to 0.9 m) wide. A rubber-tired carriage contacts the beam on the top and both sides for traction and to stabilize the vehicle. The style was popularized by the German company ALWEG.
The French company SAFEGE offers a system with cars suspended beneath the wheel carriage, and the wheels ride inside the single beam. The Chiba Urban Monorail is the world's largest suspended network.
There is also a historical type of suspension monorail developed by German inventors Nicolaus Otto and Eugen Langen in the 1880s. It was built in the twin cities of Barmen and Elberfeld in Wupper Valley, Germany, opened in 1901, and is still in operation.
Almost all modern monorails are powered by electric motors fed by dual third rails, contact wires or electrified channels attached to or enclosed in their guidance beams, but diesel-powered monorail systems also exist. Historically some systems, such as the Lartigue Monorail, used steam locomotives.
Magnetic levitation train (maglev) systems by the German Transrapid were built as straddle-type monorails, as they are highly stable and allow rapid deceleration from great speed. At speed, maglev trains hover over the track and are not in physical contact with it. The maglev is the fastest train of any type, the experimental SCMaglev having recorded a speed of 603 km/h (375 mph). The commercial Shanghai Maglev Train has run at 501 km/h (311 mph). However, the guideway is so wide that it can be argued it is not legitimate to call it a monorail. There are also slower maglev monorails intended for urban transport, such as Japan's Linimo (2003).
Some early monorails (notably the suspended monorail at Wuppertal, Germany, which dates from 1901 and is still in operation) have a design that makes it difficult to switch from one line to another. Some other monorails avoid switching as much as possible by operating in a continuous loop or between two fixed stations, as in the Seattle Center Monorail.
Current monorails are capable of more efficient switching than in the past. With suspended monorails, switching may be accomplished by moving flanges inside the beamway to shift trains to one line or another.
Straddle-beam monorails require that the beam moves for switching, which was an almost prohibitively ponderous procedure. Now the most common way of achieving this is to place a moving apparatus on top of a sturdy platform capable of bearing the weight of vehicles, beams and its own mechanism. Multiple-segmented beams move into place on rollers to smoothly align one beam with another to send the train in its desired direction, with the design originally developed by ALWEG capable of completing a switch in 12 seconds. Some of these beam turnouts are quite elaborate, capable of switching between several beams or simulating a railroad double-crossover.
An alternative to using a wye or other form of switch, is to use a turn table, where a car sits upon a section of track that can be reoriented to several different tracks. For example, this can be used to switch a car from being in a storage location, to being on the main line. The now-closed Sydney Monorail had a traverser at the depot, which allowed a train on the main line to be exchanged with another from the depot. There were about six lines in the depot, including one for maintenance.
Rubber-tired monorails are typically designed to cope with a 6% grade. Rubber-tired light rail or metro lines can cope with similar or greater grades - for example, the Lausanne Metro has grades of up to 12% and the Montreal Metro up to 6.5%, while VAL systems can handle 7% grades.