Make public transit free first. Then it will be obvious where to put the rail.

Below is an excerpt from an article by an urbanist who knows what he is talking about. Capital-intensive rail projects require density. Subways are excellent in dense areas, but the purpose of some subways is simply to reduce the pain of the auto-system and keep it viable. As long as the auto-system is heavily subsidized, no proper planning is possible. First... GET RID OF THE PRIVATE AUTO. Make public transit free. Use buses first, then light rail and streetcars as indicated. When the city is revived, the development patterns will change and it will become obvious where to put the rail.

Why Private Transit is Destined to Fail | The Mark: "...The critical factor in transit provision is density. High-density areas can support high volume transit like subways. As the density is reduced, the supportable form of transit changes from subways to streetcars to buses to minibuses to jitneys. Putting the wrong vehicle in an area is an invitation to requiring operating subsidies, or to inadequate service that will lead to declining ridership...."