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So says the Cato Institute.  A couple of the more pungent points:

4. Light rail increases congestion.

Most light-rail lines operate on streets for at least part of their length, and transit planners time traffic signals to favour trains over automobiles. The delays that result greatly exceed the benefit of getting a handful of people out of their cars.

A new light-rail line in Minneapolis so disrupted traffic signals that people using a parallel highway found they were spending an added 20 minutes or more sitting in traffic. Internal documents revealed that the government knew this would happen, but the state says it can never be completely fixed because federal rules require that signals favour the light rail.

7. Light rail increases energy consumption and greenhouse gases.

Light rail uses less energy and generates less carbon dioxide, per passenger kilometre, than buses (though not necessarily less than autos). But light rail does not replace buses; instead, transit agencies typically reroute corridor buses to be feeder buses for the light-rail line.

Many people choose to drive to light-rail stations rather than wait for a bus and then transfer to a train, so feeder buses are much more lightly used than the previous corridor buses. When Salt Lake City opened its light-rail system, the average number of people riding its buses fell by nearly 50 per cent.

When taken as a whole, then, most transit systems with light rail use more energy and emit more greenhouse gases per passenger kilometre than they did when they operated only buses. Most also use more energy and emit more carbon dioxide, per passenger kilometre, than typical automobiles.

In the rare cases where light rail has reduced energy use, the energy cost of building it swamps any savings. If we want to save energy and reduce greenhouse gases, automotive improvements such as hybrid-electric cars can do far more at a far lower cost than even the best rail projects.