Successful transit must serve density

From Excerpt: Many Cities Have Transit. How Many Have Good Transit? from Christof Spieler's, "Trains, Buses, People: An Opinionated Atlas of US Transit."

The fundamental math of density leads to an obvious rule: put transit where the people are. Successful transit needs to go where population densities are highest. Unfortunately, this is surprisingly uncommon. Across the United States, transit service often doesn’t match density well. Rather than serving density, many rail lines seem to avoid it. This figure shows multiple light-rail and commuter rail systems, at a consistent scale overlaid on density. Many follow former freight rail lines, which are often in unpopulated industrial corridors. Some were deliberately kept out of denser areas to avoid political opposition.