Class I's and most regionals use different radio channels for different districts and are controlled by microwave or fiber from the dispatch centers.

A train "keys up" a particular tower by punching in a unique 4-digit DTMF (Touch Tone) code which lights up a screen and alerts the dispatcher covering the area where it is having problems who in turn tells "Dr. Goodwrench" the train that is affected and which tower to reply on. The "heads up" from the dispatcher gives the help desk time to pull up the EMD or GE diagnostics screen and formulate a response.

Unlike a regular dispatcher's screen which covers his (as well as adjacent)regions, the motive power and signal "help desks" can access any base station on the system.

In regards to your question about interference; other trains within say a 30 mile radius will hear the conversation but tower locations and channel spacing minimize the problem.

Have a safe holiday weekend!

Dick Samuels
Oregon Pacific Railroad