vault structures beneath the rails had to be provided. They also required physical strength and skill to operate, and alert operators to avoid obstructions and other cable cars. The cable had to be disconnected ("dropped") at designated locations to allow the cars to coast by inertia, for example when crossing another cable line. The cable would then have to be "picked up" to resume progress, the whole operation requiring precise timing to avoid damage to the cable and the grip mechanism. Breaks and frays in the cable, which occurred frequently, required the complete cessation of services over a cable route while the cable was repaired. Due to overall wear, the entire length of cable (typically several kilometres) would have to be replaced on a regular schedule. After the development of reliable electrically powered trams, the costly high-maintenance cable car systems were rapidly replaced in most locations. Cable cars remained especially effective in hilly cities, since their nondriven wheels would not lose traction as they climbed or descended a steep hill. The moving cable would physically pull the car up the hill at a steady pace, unlike a low-powered steam or horse-drawn car. Cable cars do have wheel brakes and track brakes, but the cable also helps restrain the car to going downhill at a constant speed. Performance in steep terrain partially explains the survival of cable cars in San Francisco. The San Francisco cable cars, though significantly reduced in number, continue to perform a regular transportation function, in addition to being a well-known tourist attraction. A single cable line also survives in Wellington (rebuilt in 1979 as a funicular but still called the "Wellington Cable Car"). Another system, actually two separate cable lines with a shared power station in the middle, operates from the Welsh town of Llandudno up to the top of the Great Orme hill in North Wales, UK. Gas In the late 19th and early 20th centuries a number of systems in various parts of the world employed trams powered by gas, naphtha gas or coal gas in particular. Gas trams are known to have operated between Alphington and Clifton Hill in the northern suburbs of Melbourne, Australia (1886–1888); in Berlin and Dresden, Germany; in Estonia (1921–1951); between Jelenia Góra, Cieplice, and Sobieszów in Poland (from 1897); and in the UK at Lytham St Annes, Trafford Park, Manchester (1897–1908) and Neath, Wales (1896–1920). On 29 December 1886