A pretty good, short primer. Read the whole thing here: http://www.economist.com/blogs/economist-explains/2013/08/economist-explains-18
AMERICA has by far the largest rail network in the world, with more than twice as much track as China. But it lags far behind other first-world countries in ridership. Instead of passengers, most of America's massive rail network is used to carry freight. Why don't Americans ride trains?Rail ridership is usually measured in passenger-kilometres—one passenger-kilometre represents one passenger travelling one kilometre. One 1,000-person train travelling 1,000 kilometres would on its own account for a million passenger-kilometres. Yet American railroads accounted for just 17.2 billion passenger-kilometres in 2010, according to Amtrak, America's government-backed passenger rail corporation. In the European Union, railways accounted for nearly 400 billion, according to International Union of Railways data. When you adjust for population, the disparity is even more shocking: per capita, the Japanese, the Swiss, the French, the Danes, the Russians, the Austrians, the Ukrainians, the Belarussians and the Belgians all accounted for more than 1,000 passenger-kilometres by rail in 2011; Americans accounted for 80. Amtrak carries 31m passengers per year. Mozambique's railways carried 108m passengers in 2011.