Often, and in most metropolitan areas, transit systems are built by "gradual accretion" over the years. Tacking on individual requests rarely leads to overall efficiency, and systems often grow more complex, less and less legible for customers and unmarketable.
Making transit systems more fluid and attractive involves considering a host of factors, such as evolving districts and their new demographics, of course, and the creation of commercial or industrial infrastructure. Developing a business district, for instance, requires redesigning the transit system to adequately serve the new area.
