Port Authority contractors today activated the large tunnel-boring machine in a pit below Stanwix Street to start digging the second tunnel that is part of a $435 million, 1.2-mile light-rail extension to the North Shore.

Spokesman David Whipkey said the project remains on schedule and the tunnel is expected to be completed by the end of the year.

The machine finished boring the first tunnel under the Allegheny River on July 10. Workers have spent the time since then turning it around to head in the northbound direction, doing maintenance and building a tunnel "seal" to pressurize the portal.