A streetcar named "Washington DC"

Ryan Avent, DC transit blogger extraordinaire, says of the beleaguered Metro system:

A good first step would be–surprise!–a new cross-town line, especially if that line were built to handle two lanes in either direction. That should help prevent stoppages in the core from rapidly propagating throughout the system. A good second step would be to augment Metro with better streetcar and bus systems, shifting some local traffic off Metro and ensuring the availability of other options should the need for alternatives arise.

I don't get the streetcar fetish. Philadelphia had them, and they always struck me as having all of the downsides of busses, with none of the advantages. To be sure, they were fine in the days when internal combustion engines had very low efficiency, but those days are long gone. What am I missing?