? I always thought Muni metro was exceedingly productive. It has the 3rd highest lightrail ridership per mile in the United States (http://en.wikipedia.org/wiki/List_of_United_States_light_rai...).

As any rider can attest, it gets pretty busy. Perhaps you are thinking of SF's southern neighbor, San Jose, as an example of a highly underutilized light rail system?

Also, a lot of muni metro is underground, which is essential for keeping in fast beneath the high traffic volumes above.