I was reading up on cable cars, and it struck me as odd that they only use one cable. While the system isn't likely to fail, and there are contingency measures (e.g. brakes) in case something happens while a car is going downhill, it seems odd that the designers of, say, the San Francisco cable car system didn't add in at least a second cable.
Why is this?
My guess is because of turning (explained here) - if there were two cables, one would have to go faster for a given turn (unless the "let go" method was used).