Most new rail transit lines in the United States and around the world are either light rail, including lines that sometimes run in or cross city streets, or heavy rail, which are built in exclusive rights of way, usually elevated or in subways. Heavy rail costs far more to build than light rail, but the capacity of light rail to move people is far lower than heavy rail. In fact, the terms light and heavy refer to people-moving capacities, not the actual weight of the equipment.
Recently, a number of cities in the United States and elsewhere have built or are building a hybrid form of rail transit that can best be described as the worst of both, combining the cost-disadvantages of heavy rail with the capacity limits of light rail. Seattle is building a three-mile subway that costs nearly six times as much per mile as the average light-rail line. Honolulu is building a 20-mile elevated rail line that costs well over twice as much as the average light rail. Yet those lines will be limited to little (or no) more than light-rail capacities. This seems to be a worldwide trend, as new, high-cost, low-capacity rail systems have recently opened in Mumbai, India; Panama City, Panama; Fortaleza, Brazil; and several other Asian and Latin American cities. A small number of French, German, Italian, and Spanish contractors and railcar manufacturers seem to be involved with building and supplying many of those lines.
Rail lines built at light-rail costs are questionable enough, as in nearly every case buses can move more people just as comfortably (if not more so), just as fast (if not faster), and at a far lower cost. Buses share infrastructure with cars and trucks, reducing their cost, while the use of high-occupancy vehicle or high-occupancy toll lanes would allow buses to avoid congestion during even the busiest times of day.
The willingness of many rail advocates to support high-cost, low-capacity rail lines calls into question the entire rail agenda. Supporters of low-capacity lines are not truly interested in transportation; supporters of high-cost lines are not truly interested in urban efficiencies. If they are not willing to draw the line against such projects, then there is little reason to believe their claims about the benefits of other rail projects.