The line continues west from Lake Oswego to a location known as "Cook", where the Tigard branch splits off to the northwest.

The Newberg branch continues southwest, crossing both the Tualatin River and the former main line of the Oregon Electric Railway.

The Newberg branch was built to connect a network of 3 ft (914 mm) gauge railroads in the Yamhill Valley to Portland.

[4] In Dundee, it connected with the Oregonian Railway, also a narrow gauge road, whose main line continued southwest to Sheridan and Airlie.

[4] In 1905, the Southern Pacific built a new cutoff from Dundee to Saint Joseph via Lafayette, connecting the line with the West Side branch.

[8] Services began at Portland Union Station, then used either the Newberg or West Side branch to reach McMinnville and then Corvallis.