Tower 79 at Paris, Texas
I was doing some research on the railroads of Paris, Texas, and found the following article (Page 363) in the magazine Railway Signaling. It has a detailed article about the 1936 installation of an automated system that replaced the staffed system from 1909. It also has a nice diagram of the location.
The article explains why the Frisco was included, even though it didn't cross the main diamond. The Frisco had an industrial line (Compress Line) that crossed the Southern Pacific line just north of the T&P diamond, and it was included in the interlocking.
You cna find the article at: