While I don't know the exact date, the line north from Snohomish to Burlington, WA., was removed from service fairly soon after the BN merger. The line from Burlington north to Sumas, WA. and the Canadian border remains in service, though downgraded from its' glory days as Northern Pacific's main Canadian connection. BN had the luxury of various NP and GN lines when piecing together their Pacific Northwest mainlines; the GN lines being newer and better engineered usually won out.