SAN FRANCISCO (CBS/AP) – Federal officials have given San Francisco the green light to proceed with major construction work for its $1.6 billion subway expansion project.

Mayor Ed Lee said Wednesday the city received a “letter of non prejudice” from the U.S. Department of Transportation. He told the San Francisco Chronicle the letter is a strong sign that officials will approve nearly $1 billion in federal grants for the expansion within the next few months.

The project would extend the T-Third line from the Caltrain station, on the south side of the city, to Chinatown, on the north side.

City transportation officials say the letter will allow them to start digging a large hole near the Caltrain station where crews will lower the tunnel-boring machine.

The subway’s estimated opening date is 2019.

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