The massive machines consist of a rotating cutter wheel (the cutter head), a cylindrical steel shell (the shield) and a 300ft (91m) train of tunnel-building mechanisms (the trailing gear). Starting a few weeks from now, Mom Chung will excavate and construct the southbound tunnel, digging at a rate of about 40ft (12m) per day far beneath the surface of the city. Her twin sister, Big Alma, will begin building the tunnel for northbound trains about two months later.
