Workers inspect the “shock tubes” that set off blasting caps, which detonate the charges. More than 2,400 blasts have been conducted on the project. | Photo: Dean Kaufman

This tunnel is under 55th Street and Park Avenue. The yellow material is a waterproof membrane, which goes on before the concrete finish. | Photo: Dean Kaufman

During the day, rail cars will wait under Grand Central Terminal until it’s time to take commuters back home to Long Island. | Photo: Dean Kaufman

The concrete floor of the 55th Street ventilation chamber being poured. A total of 10 such areas will circulate air between the tunnels and the surface. | Photo: Dean Kaufman

This shows an access tunnel near 50th Street. These cross-passages (there are 19 total) can be used in emergencies or when repairs are needed. | Photo: Dean Kaufman

In what will be the LIRR terminus under Grand Central, workers use a hose with a pneumatic nozzle to spray Shotcrete onto the tunnel walls. | Photo: Dean Kaufman