Freight Symbol Books
Dispatcher Sheets for April 2, 1949
The Maine Central Railroad History and Modeling Page has a list of freight assignments from 1958.
The Boston and Maine Historical Society has 1922 and 1941 employee timetables online, but unfortunately these do not provide much information on freight operations.
Melvin Photos offers a data CD that contains freight assignment listings for a variety of years similar to the Maine Central Railroad History and Modeling Page above.