So you say you want a subway. Elon Musk has some ideas for how to do that, though as Alon Levy argues in Chicago today, there's reason to be skeptical that Musk's ideas for disrupting traditional subway construction will save as much as the entrepreneur assumes.
Shaving off construction costs is an old practice in subway construction, as is clear from this fascinating old film Streamlining Chicago, which gets into the details of how the underground section of the 'L' was built. (The film isn't dated, but the city broke ground on it—with considerable federal assistance—in 1938 and the State Street tunnel was completed in 1943, due to delays from World War II.)
The interesting part of the film, after a few minutes of throat-clearing about the greatness of progress, begins with the State Street tunnel. The 200-foot-long, 6,000-ton twin concrete tubes were built, sealed, and put in what was essentially a giant wooden box, then shipped up the Calumet River and Lake Michigan and into the Chicago River. Then they were sunk into place instead of being built into a tunneled cavern under the river, allowing just five feet of clearance between the tunnel and the riverbed.
From there it moves north to North and Clybourn and two more forms of subway construction: cut-and-cover and open-cut. Between North/Clybourn and Fullerton is where the Red Line goes from underground to elevated, and it's a pretty straightforward process: dig a hole and cover it.
A bit later on you can see why tunneling was to be avoided when it could. The soft clay beneath downtown had to be scraped off by a knife, which was positioned by hand and then pulled by a winch, peeling off long strips of clay, piece by piece, about 20-30 feet per day. Sadly not captured by the film is the "biscuit cutter," a 225-ton steel contraption loaded with 24 hydraulic jacks, which used 33-inch steel rings to remove 129,000 pounds of clay with each push—and passing skyscraper foundations by just a few feet.