Starting next year, two TBMs will build the tunnels, digging at a rate of about 40 feet per day far beneath the surface of the city. Prefabricated tunnel segments will be put in place as the TBMs advance, with a crew of about seven workers operating the machines and welding the segments together.

Each TBM consists of a rotating cutting wheel (the cutter head), a cylindrical steel shell (the shield) and a 300-foot train of tunnel-building contraptions (the tail). It takes about a year to construct this complex construction equipment.

Previously, previously.