The Google Maps API allows you to display the public transit network of a city on your map using the TransitLayer object. When the Transit Layer is enabled, and the map is centered on a city that supports transit information, the map will display major transit lines as thick, colored lines. The color of the line is set based upon information from the transit line operator. Enabling the Transit Layer will alter the style of the base map to better emphasize transit routes.
Transit information is only available in select locations.
The following example shows the Transit layer enabled on a map of London, UK:
Google reference: Transit Layers Reference