After more than three weeks of delay, the behemoth machine boring the first of two transit tunnels under the Allegheny River is back in operation.

Crews restarted the 500-ton machine yesterday morning, Port Authority officials confirmed. By this morning, the machine was expected to be about 60 feet below the water surface just off the north shore of the river.

The contractor stopped the machine in mid-April, about 10 feet short of the river, when a large tree trunk with roots somehow became lodged at the front of the machine.

A consortium of West Mifflin-based Trumbull Corp. and San Francisco-based Obayashi holds a $156.5 million contract to bore the twin tunnels for the Port Authority's 1.2-mile light-rail extension from Downtown to the North Shore, which will be served by two stations.

The overall project still is estimated to cost $435 million, despite inflationary pressures, and be running in early 2011.

Readers can follow the progress of the tunneling at www.boretotheshore.com.