There are many different ways of structuring a set of transit routes into a network to provide transit service to a given area. These might include any combination of:
Radial services, focusing on collecting passengers from outlying areas and bringing them into a major trip generator (e.g., a downtown area, major employment center, or other significant destination);
Cross-town or grid-like routes, focusing on connecting passengers across the area, perhaps between radial services or among several smaller trip generators; and,
Direct connections, focusing on moving passengers between major trip generators (e.g., between downtown and a high-density residential area, or between a major employer and downtown).
Circulators, focusing on collecting and distributing passengers in smaller sub-areas.
Usually, transit networks are made up of some combination of these types of routes, allowing a satisfactory level of access to most of the region, with a high degree of direct service to major trip generators. Sometimes, this may involve a combination of modes. For example, in Boston, the rail systems are strongly radial, while the bus system provides cross-town and circulator services. The scale image below shows the strong radial nature of these lines, directed to downtown Boston.