Monorails are a transportation technology developed in the middle of the 20th Century. Famously exhibited at the 1964 World's Fair in Seattle, monorails have largely seen use in theme parks and airports for shorter-distance rapid transit.
Monorails are typically fixed-length, multi-unit vehicles with rubber tires ride on a concrete "rail," or more precisely, a concrete guideway. The shape of a monorail vehicle, itself, often appears to straddle the guideway.
Power for the trains is delivered via two contact rails at voltages typical for other kinds of rail transit. Monorails can be operated by a motorman or, more commonly today, are fully computer-operated.
Monorails have a smaller footprint, infrastructurally, than heavy elevated railways. Overall, construction may involve single support beams between spans of a thick, concrete rail, with mounted utilities such as power delivery and communications systems.
Due to the use of rubber tires, the ride is smooth for riders, and the high-traction . There also is little noise produced by a monorail train in operation.
Monorail systems do not appear to have a defined, standard set of specifications, and are typically custom-designed, with custom parts, signals, and control systems for each existing installation.
Because a monorail is essentially a vehicle operating in a double-flanged fashion over a single rail, switching trains is generally a larger infrastructural challenge. Rails cannot cross each other or intersect in any way. As a result, monorail switching systems will often involve large contraptions to dismount, disconnect from, and realign a large section of movable guideway to connect to another segment. Other solutions include multi-ton pieces of fixed monorail infrastructure on each side of a pivoting platform, which can be flipped completely over to guide trains over one of two possible route selections.
While these hefty switching mechanisms may be acceptable for smaller systems where trains rarely need to be switched from track to track. For example, the short, demonstrational Seattle monorail has one train per track and each just go back and forth on each without any switching. A heavily-used urban transit system generally requires one or more tracks dedicated for each direction, with trains entering terminals every few minutes and being switched back out onto the opposite track after changing ends for the return trip.
An alternate solution to this problem is building a large "loop track" to allow trains to pull past the station, come back around a looping track, and return to the terminal on the outbound side--however, this requires large terminal areas and is impractical for a simple stub-end terminal as is most practical in already-developed areas (larger terminal infrastructures then require costly property acquisition and demolition decreasing density and breaking up neighborhoods).
The use of rubber tires requires a high amount of vehicle maintenance to prevent or replace worn or blown out tires, and potentially increased friction (thus decreasing energy efficiency, even though tires are likely to be automatically kept at ideal pressure ratings).
While they often use more modern technologies than older electric railways, any of these technologies could be applied to more standard light or heavy rail systems (provided they have a separated, private right-of-way).
There are a number of nice things about monorails, but they are generally not selected as a technology for urban transportation systems due to the proprietary nature of technologies, incompatibility with standard railway equipment, and practical limitations on system capacity and flexibility.
Transit systems to carry large numbers of people over greater distances almost always opt for flanged steel wheels on steel rails, in the form of streetcars, light rail trams, or heavy rail rapid transit (subways/elevated lines).
Monorail use is generally, then, limited to shuttle services around airports, theme parks, and resorts areas. Here are a few places where monorails exist today:
See also: Wikipedia: Monorail