Components of an Urban Transit System

The above figure represents an hypothetical urban transit system. Each of its components is designed to provide a specific array of services. Among the defining factors of urban transit are capacity, frequency, flexibility, costs and distance between stops:

Metro system. A heavy rail system, often underground in central areas (parts above ground at more peripheral locations), with fixed routes, services and stations. Transfers between lines or to other components of the transit systems (mainly buses and light rail) are made at connected stations. The service frequency tends to be uniform throughout the day, but increases during peak hours. Fares are commonly access driven and constant, implying that once a user has entered the system the distance traveled has no impact on the fare. However, with the computerization of many transit fare systems, zonal/distance driven fares are becoming more common.
Bus system. Characterized by scheduled fixed routes and stops serviced by motorized multiple passengers vehicles (45 - 80 passengers). Services are often synchronized with other heavy systems, mainly metro and transit rail, where they act as feeders. Express services, using only a limited number of stops can also be available, notably during peak hours. Since metro and bus systems are often managed by the same transit authority the user's fare is valid for both systems.
Transit rail system. Fixed rail comes into two major categories. The first is the tram rail system, which is mainly composed of streetcars (tramways) operating in central areas. They can be composed of up to 4 cars. The second is the commuter rail system, which are passenger trains mainly developed to service peripheral / suburban areas through a heavy (faster and longer distances between stations) or light rail system (slower and shorter distances between stations). Frequency of services is strongly linked with peak hours and traffic tends to be imbalanced. Fares tend to be separate from the transit system and proportional to distance or service zones.
Shuttle system. Composed of a number of privately (dominantly) owned services using small buses or vans. Shuttle routes and frequencies tend to be fixed, but can be adapted to fit new situations. They service numerous specific functions such as expanding mobility along a corridor during peak hour, linking a specific activity center (shopping mall, university campus, industrial zone, hotel, etc.) or aimed at servicing the elderly or people with disabilities.
Paratransit system. A flexible and privately owned collective demand-response system composed of minibuses, vans or shared taxis commonly servicing peripheral and low density zones. Their key advantage is the possibility of a door-to-door service, less loading and unloading time, less stops and more maneuverability in traffic. In many cities of developing countries, this system is informal, dominant and often services central areas because of inadequacies or high costs of the formal transit system.
Taxi system. Comprises privately owned cars or small vans offering an on-call, individual demand-response system. Fares are commonly a function of a metered distance/time, but sometimes can be negotiated. A taxi system has no fixed routes, but is rather servicing an area where a taxi company has the right (permit) to pickup customers. Commonly, rights are issued by a municipality and several companies may be allowed to compete on the same territory. When competition is not permitted, fares are set up by regulations.

The table below shows the interval between access points for public transit modes:

Urban Transit Mode	Distance between stops / stations
Metro / Subway	0.5 - 2.0 km
Bus on conventional road network	0.2 - 0.5 km
Bus on reserved lane or express bus	0.5 - 1.6 km
Commuter rail	3.0 - 10.0 km
Light rail	1.0 - 5.0 km