Image from John W. Barriger III National Railroad Library flickr page.
GN Condensed Freight Schedule 14 12-1-1965
GN Condensed Freight Schedule 16 1-1-1968
The GN-NP Joint Archive has a huge collection of employee timetables. There are also dispatcher sheets and other operational materials located on their Cascade Division page.
Wx4.org also has a large collection of employee timetables, especially here and here.
Freight train consists can be found at Great Northern Empire Then and Now and also in the files section of the Ry-ops-industrialSIG group.
The Great Northern Railway Historical Society has a nice collection of articles describing freight train operations, TOFC traffic, and iron ore traffic available for purchase here.