It was the invention of the cable car that made high-society life on San Francisco's hills possible and practical. Since 1873, these little trolleys have been an integral part of life in the city, thanks to Scots-born Andrew Hallidie's concern for horses. Having watched a team struggle and fall, breaking their legs on a steep San Franciscan street, Hallidie designed a pulley system around the thick wire rope his father had patented in England. A transportation revolution followed. At their peak, just before the 1906 earthquake, over six hundred cable cars traveled 110 miles of track throughout the city; over the years, usage dwindled and, in 1964, nostalgic citizens voted to preserve the last seventeen miles (now just 10) as a moving historic landmark. Today there are three lines. Two of them, the Powell & Mason and the Powell & Hyde lines run from Hallidie Plaza off Union Square at Powell and Market streets to Fisherman's Wharf. The Powell & Hyde is the steepest, reaching a hair-raising 21-degree grade between Lombard and Chestnut streets. The oldest route, the California line, climbs Nob Hill along California Street from the Embarcadero, rattling past the fanciest hotels in the city. The cars fasten onto a moving two-inch cable, which runs beneath the streets, gripping on the ascent then releasing at the top and gliding down the other side. You can see the huge motors that still power these cables in the Cable Car Museum and Powerhouse , at 1201 Mason at Washington Street (free; daily 10am-6pm; ).