We have to be careful not to run “rail lines to nowhere”, of course. Amtrak is now running 90% full most of the time. Most well-managed new light rail lines run at least 40% full. At those percentages they are more efficient than cars. Any place a two-lane-each-way freeway is crowded can support passenger rail, and it’s cheaper than freeway widening.

For busy routes, examine the number of people (and goods!) travelling it. For high volume travel, rail is far more efficient. When full six lane superhighways need repairs, replace them with rail. (Roads are great for low volume). This ‘movement density’ number also shows you if you want a streetcar or a subway. Japan has a chart for this; we should too.