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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).

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    $\begingroup$ What would the benefit of a second cable be? Balance that against the huge increase in complexity, coupled with the likely overall decrease in system reliability. $\endgroup$ – Dave Tweed Mar 23 '15 at 1:59
  • $\begingroup$ @DaveTweed I hadn't considered that. At the same time, though, one could argue that any redundant measure adds a host of complexity. Sure, there are more things that could go wrong, but unless some event took out both systems, one would keep operating. $\endgroup$ – HDE 226868 Mar 23 '15 at 2:01
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    $\begingroup$ Increased complexity also means increased costs: capital, operating & maintenance $\endgroup$ – Fred Mar 23 '15 at 5:08
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    $\begingroup$ Think of it like this: even ski lifts only use one cable. So even without additional safety measures (like brakes and not having a gaping void underneath the cars), one cable is apparently reliable enough. $\endgroup$ – Sanchises Apr 10 '15 at 21:02
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I wouldn't be surprised if the brakes on a cable car had multiple contours. But, I don't see why brakes alone would be insufficient. The brakes are sufficient for stopping at traffic lights.

If the gripping mechanism fails, then the cable will break and stop. A team of horses will tow the cable car to the depot.

Having said that, SF cable cars are not the safest public transport. The system was designed in 1890s when safety concerns were more relaxed.

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