Dick Carpenter has written four volumes published by John Hopkins University press called: "A railroad atlas of the United States in 1946" Mr. Carpenter has Volume 5,Iowa and Minnesota,out now. They show all the interlockings in service in that year.
Mike Condren has a site that concerns with railroading in the Memphis Tennessee area. There is a section on towers that will interest you. condrenrails.com
The TrackSide Photographer is another "Ironman" approved site. The focus is not on trains per se but other parts of railroading;bridges,structures,track,equipment. Great site..... thetracksidephotographer.
Harrisburg PA. NRHS has done a great job of restoring Harrisburg tower. Here is the link to their site.