The solution was to lay down another pair of tracks for Muni, placed outside the United Railroads tracks from the Ferry Building to Castro Street. By 1918, eight Muni lines used those tracks, making Market Street, along with New Orleans’ Canal Street, the only US main streets with such a four–track arrangement.

In the 1920s, San Francisco’s landmark Ferry Building was the busiest transportation terminal in America—behind only London’s Charing Cross Station worldwide—with more than 400 streetcars swinging through the triple loop in the 90–minute evening rush hour to serve the 43 ferryboats that tied up there. At stops along Market, barely two feet separated streetcars on the inside and outside tracks, forcing passengers waiting for cars on the inside tracks to squeeze sideways as cars rumbled by.

People from all around the Bay Area came to Market Street to shop at department stores like the Emporium, Hale’s, and Weinstein’s. They came to see movies at the Warfield, the State, and the grandest of all, the Fox. They came to visit their doctors and dentists at the Flood Building, and for dozens of other reasons. Market Street was one of the busiest streets in the world.

The opening of the San Francisco/Oakland Bay Bridge in 1936 changed the scene. More automobiles descended on the city. When interurban trains began running on the bridge in 1939, ferry traffic declined further and many streetcars were diverted to the Transbay Terminal at First & Mission Streets. World War II rejuvenated Market Street and its streetcars as soldiers and sailors shipped in and out, but soon after the War ended, so did the fabled ‘Roar of the Four’.