Rail transit
Trains are vehicles that operate on tracks.
In transit/railroad terms, a car means a single rail vehicle. A train means multiple rail cars connected together (2-car trains, 5-car trains).
Passenger trains vary in speed and distance. Some trains in the East Coast and abroad can operate faster than 125 mph connecting major cities. Some rail lines in the Bay Area function like a local bus with stops every few city blocks.
Below are the basic passenger train types. These categories, however, are shades of gray because a train system may have more than one function (Amtrak inter-city trains serving commuters). Also, train systems that have similar functions can have major differences in underlying technologies and legal requirements. See BayRail Alliance for alternate explanations.
High speed trains
High speed trains are generally defined as trains that can operate 125mph or faster. High speed trains generally connect large metropolitan areas (with very few stops in between) and are competitive with airlines in terms of overall travel time.
There's a proposal to construct a high speed rail system between the Bay Area and Southern California via the Central Valley.
Inter-city trains
Inter-city trains generally mean trains traveling long distances connecting metropolitan areas. Although the distances covered by some of these trains are comparable to airlines, inter-city trains generally operate at highway speed. Long distance inter-city trains may provide amenities not found on most other forms of transportation, including sleeper-cars and cafe/dining cars.
Amtrak is the operator of inter-city trains in the United States.
Although riding Amtrak is much slower than flying, inter-city trains are nonetheless valuable. Between metropolitan areas, Amtrak trains serve small cities that aren't served by airlines.
Commuter trains
Commuter trains generally mean trains connecting suburban areas with the central city and primarily serves riders to and from work. Commuter trains typically run on weekdays, during rush hours, and only in the peak direction. A prime example would be Altamont Commuter Express, which run from Stockton to San Jose during weekday mornings, and from San Jose to Stockion during weekday afternoons.
Commuter trains however can run more service than what the category typifies. Caltrain for instance operates 7-days a week with all-day train service in both directions. Many also considers BART to be commuter rail because of suburbs the system serves.
Rapid transit
Rapid transit, which is also known as metro, subway, and heavy rail, mean trains that generally serve the urban-core, have large passenger capacity, and operate totally separate from road traffic. In order to run separately from road traffic in the city-core, rapid transit trains would run either above or underground.
Many major cities (like New York, London, Washington D.C.) have extensive systems that make traveling within a city fast and convenient. BART is the rapid transit system in the Bay Area. However, BART does not serve San Francisco as extensively as other systems do in their cities.
Light rail
Light rail, which might be also known as trolley and streetcars, mean trains that function as local transit in an urban-core and can operate on the street-level. Compared to rapid transit, light rail costs less, is more pedestrian friendly, but has less passenger capacity.
In Sacramento and San Jose, light rail trains run faster in the suburbs (dedicated tracks) and slower in downtown (street median). In San Francisco, trains operate in mixed traffic outside downtown and underground in downtown. Light Rail stations can be a few city blocks apart in downtown and a mile or more apart in the suburbs.
Generally fares for light rail are the same as the buses.
Light rail are modern versions of the trolleys and streetcars before the age of automobiles and buses. San Francisco and San Jose feature services provided by historic vehicles. San Francisco's cable cars function very similar to light rail except that the vehicles are pulled by underground cables.
