San Fran underground

On one of my trips to San Fran, I happened to pass by a subway entrance one evening while heading back to the hotel after shooting a bit.  I find subways interesting because we don't have them here in Texas - too much rock to dig through I guess.  I also enjoy shooting trains so I headed on down the steps to underground San Francisco.  It's cool being down there, especially at night when it is empty.  I really enjoy the distant rumble of an approaching train, and the squeak of the brakes when it gets there.  Aren't those just distinctive sounds?