The agreement announced Monday completes more than six years of talks between Seattle officials and the railroad over a stretch of track from the industrial area of Renton, Wash., south of the port to the agricultural region around Snohomish, northeast of Seattle.
BNSF had donated the southern portion of the north-south corridor to the city and the new agreement covers the northern section, beween Woodinville and Snohomish.
"The port's goal has always been to preserve the corridor and place it in public ownership and we've accomplished that goal," Port of Seattle CEO Tay Yoshitani said in a statement. "I'm grateful to BNSF for their willingness to work with the port, and to our partner agencies for joining us in the effort."
The port said it would maintain rail service on the line with a third-party operator. Local reports said GNP Railway and Ballard Terminal Railroad, which has operated a short line BNSF abandoned in 1997, will run the spur.
Seattle said it would also use parts of the line for transportation and recreation, and officials in King County have said they may use the track rights to add a light rail line.
BNSF has been looking at selling the track since 2003, and local reports said the port at times was close to buying the line for more than $100 million.
"BNSF is very pleased that the transaction is complete, and that the Puget Sound region will benefit from the corridor," BNSF spokesman Gus Melonas said in a statement.
Regional utilities PSE and CWA will negotiate easements for their facilities and services along the corridor - easements that are crucial for existing and future service and growth.