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Mass transit (aka public transportation) is the deployment of transportation systems (usually trains or buses) meant to carry a large number of people from place to place within a geographical (usually urban or suburban) area. Especially in densely populated areas, well-planned mass transit systems vastly reduce the need for personal cars and the attendant auto exhaust pollution and wasted space used for parking lots. Mass transit systems are usually run by or contracted by local government agencies  to provide convenient short and medium-range service for commuters and tourists. In many parts of the world, at least the most developed parts, public transportation has been a major part of city planning doctrine since the late 19th century, though in much of the United States, recent urban development has been done to more fully accommodate automobiles, reducing the demand for trains and buses. However, with rising gas prices and millennials increasingly rejecting car culture, mass transit is making a comeback even in the US, a trend that most of Europe has already had since the oil crisis of 1973. Amtrak for example has posted record ridership in 12 out of 13 years since 2000 and ridership has risen by over fifty percent.
Common modes of transit
- Bus: The most visible and most widely used form, the typical bus runs on a diesel (though increasingly on compressed natural gas (CNG) and electric) engine and usually operates on public streets (although there are a few Bus Rapid Transit systems that use dedicated busways). In practice, this can be anything from a privately-owned shuttle van all the way up to articulated and double-decker city buses carrying over a hundred people during peak service times.
- Trackless trolley: A type of electrically powered bus that draws power from an overhead power line (sometimes called a catenary) by way of a set of trolley poles. Although confined to what is essentially an overhead track, trackless trolleys can reduce emissions even more with centralized power generation. Certain types of buses, styled after old-style streetcars, are also called trackless trolleys, but are built on the same platforms as a typical diesel, CNG, or hybrid bus; these are more often used for tourist rides than commuting and tend to be privately owned.
- Streetcar: Also referred to as "tram(way)s" or, especially for older systems, "trolleys". These are rail vehicles that run on tracks that are laid directly in a public street. Compared with buses, streetcars tend to have larger capacity and they have no local emissions since they run on electricity. Running on rails also gives passengers a smoother ride. The need for rails make streetcar systems costlier to build. 
- Light rail: "Light rail" refers to streetcar-like vehicles that run on a separate track away from the public streets. By having a separate track, light rail avoids traffic congestion and may therefore be faster and more reliable. The name is something of a misnomer (modern light rail cars are seldom much lighter than their heavy rail counterparts), but the name is still appropriate since light rail systems generally require less disruptive development than heavy rail (including less dedicated right of way and power from an overhead catenary). Outside the US almost all new street level rail lines, even additions to existing systems, are up to light rail standards even if called trolley or tram.
- Cable cars and funiculars are a special case of light rail where the cars are not directly powered, but pulled by a cable or screw laid at track level. Funiculars are used to climb steep hills at ski resorts and the like; very few cable car systems still exist, and the only one still regularly used for public transportation is the famous system in San Francisco, California.
- Heavy rail for mass transit tends to take any of various forms:
- Subways and elevated systems generally use large passenger cars with open-plan seating and doors that open on the same level as the station platforms, vastly simplifying access for handicapped people. These trains are almost always electrically-powered, usually from a charged third rail supplying high-voltage electricity. With a few exceptions (large swaths of Chicago for example), electric heavy rail is almost always built underground in urban cores.
- Commuter rail is any heavy rail service that primarily connects the suburbs of a city with the city center. There are many forms of commuter rail, but they are almost always heavy-rail. North American commuter rail usually uses heavy-duty diesel rolling-stock, and usually operate at lower frequencies. In many other places, they are operated with electric multiple units, and run at higher frequencies. In German speaking Europe - and by extension some other countries - commuter rail with more stops than regional trains but less than subways and the like is often called "S-Bahn" with level boarding and electric traction as well as frequent departures (15 or 30 minute headway in peak times) common
- Cross-country rail is similar to commuter rail, but often includes amenities like comfortable seats, beds, lounges and dining cars that would be impractical or pointless on a commuter train.
- High-speed rail -- Developed in the mid-20th century (the original was the Japanese shinkansen "bullet train", which went online in 1964), high speed rail systems like the French TGV the German ICE and American Acela Express are designed to cut travel times for cross-country rail trips. Most high-speed rail systems are electrically powered, often from overhead catenaries like light rail.
- Maglev -- A particular form of of high-speed rail (though it does stretch the definition of "rail") which uses magnetism to levitate the train above the ground slightly and be forced forward by linear motors rather than using wheels; this eliminates all rolling resistance. Theoretically this should make them more efficient than conventional high-speed trains, however in practice they are only more efficient above 200 mph.
- Monorail: Somewhere between light and heavy rail in terms of footprint, monorail systems usually use low-footprint overhead single tracks, either mounted directly on the track supports or put in an overhead design with the train car suspended. Monorail systems are used here and there throughout the world (especially in Europe and east Asia, particularly Japan), but apart from public transit installations in Las Vegas and Seattle, most North American monorails are either short shuttle services or privately owned services (the Disney monorail systems used at their parks may be the most famous in the world).
- Freight rapid transit: essentially a type of heavy rail system that would carry shipping containers rather than passengers beneath a carless city. The plan is largely theoretical and has not been implemented even on a pilot basis anywhere in the world. However, streetcars have historically been used for freight and some are used for freight even today. An example is the Dresden tramway that supplies car parts to a VW plant downtown.
Well-run public transportation systems with low-emissions and electrically-powered equipment reduce the number of fossil fuel powered vehicles on the road, as well as reducing overall traffic congestion within a city by concentrating commuters onto fewer vehicles. They also allow greater development density, thereby helping to make a city substantially more walkable, a net positive from a public health standpoint.
Heavily trafficked bus and train stations often create public meeting places (or serve existing ones), with commuter services such as snack bars, cafés, and copy shops within short walking distance for worker and tourist convenience, as well as opportunities for buskers to provide entertainment.
Park and ride stations in suburbs provide easy access to bus and rail facilities for commuters who live in outer suburbs and other spread out areas that may be impractical for full service. Such garages and lots also serve as easy satellite parking for large city events where access to the urban core might be otherwise cut off by roads being used as walkways for large numbers of people.
High-speed rail can compete with short-haul air travel, as it can be faster if the time spent travelling to and from the airport/train station and the time spent waiting for the vehicle to arrive are taken into account. As high-speed rail is usually fueled by electricity, which can be generated in renewable ways, carbon emissions and other negative effects of the consumption of petroleum for airplanes can be drastically reduced or even eliminated.
Another great benefit for the traveller is that train stations are usually in the center of town and equipped with a wide selection of stores, restaurants, and services, at usually lower prices than comparable services at airports. Thus a long layover is not as costly with a train as it would be with airplanes.
Many tourists also prefer taking the train, as the ground level views of the landscape are often not possible to experience by any other way of transport. Compared to a car, commuters and tourists don't have to focus on the road and can either work, sleep, or engage in leisure activities during the trip.
Perhaps more than any public works apart from highway construction, rail transit can be massively disruptive in terms of right-of-way takings and visual impact on an area, and underground systems in particular can be outrageously expensive and troubled. In addition, routing on both public streets and dedicated rights of way can be subject to significant political pressure on economic, aesthetic, or just plain NIMBY grounds. 
In addition, in many places passenger rail is seldom profitable, so in areas with relatively low public revenue (especially exurban and rural areas), keeping a transit system operational even with just buses is often very difficult and draining on government coffers. This may result in reduction of routes or schedules, such as one hour apart and no service at night or on Sundays, and car-free commuters may end up having to take an inferior job in order to accommodate reduced transit service.
Another big problem is the number of stops. If there are too many stops, service becomes too slow and inefficient as acceleration and braking cost a lot of energy. If there are too few stops, fewer people can use the service as no station is within comfortable distance. Usually the solution to this problem is to provide express services only calling at major urban centers and slower services that connect small and intermediate stations to said urban centers. However due to federalism and to alleviate NIMBY-concerns, sometimes express services stop too often and at rather unimportant stations. A good example of this is the German high-speed ICE that call in Limburg and Montabaur, two small towns less then 50km from each other, because of pressure applied by local politicians.
In the US the biggest problem is that certain people seem to have made hating public transit a part of their political and social identity. Thus getting any money appropriated for new construction or even upkeep is an uphill battle.
- Examples: Metropolitan Transit Authority (New York City); Chicago Transit Authority; Régie Autonome de Transport Parisien (RATP) and Société Nationale de Chemins de fer Français (SNCF), both in Paris; Transport for London; Massachusetts Bay Transportation Authority (Boston), Bay Area Rapid Transit (San Francisco); Berliner Verkehrsbetriebe (BVG; Berlin); Rhode Island Public Transit Authority
- The iconic old-style streetcars were one of the most common varieties of urban public transportation in the early 20th century, until automotive corporations made deals with local authorities in many cities to replace streetcar systems (some of which were suffering from flagging ridership) with bus routes.
- cf. The MBTA commuter rail's Greenbush line, opened in 2007 on an old privately owned passenger line, is one example of the last -- many residents of the town of Hingham (one of the Greenbush line's beneficiary towns) opposed it, partly on aesthetic grounds, partly on an argument that there hadn't been rail service in the area so long that people were literally too stupid to know what to do at a railroad crossing.
- Hours of operation of Citilink in Fort Wayne, Indiana, USA