The unmanned ones are actually controlled from the lead locomotive cab. They use a form of remote control. The front engineer is controlling them at the same time (s)he’s running the front engines. They have to be separate controls because the push and pull of the locomotives can cause real problems if they slam the slack in and out too hard.
Some DP (distributed power) set-ups utilize a Harris box. The UP uses this configuration as well as set-ups that are in the computer screens. The BNSF utilizes only the computer screen set-up. The engr. can have both units operating in conjunction with the lead locomotive. He (she) can also “set up the fence.” This configuration allows independent operation of both units. This is used primarily on steep, slow hills when you want the dp unit shoving harder than the lead locomotive is pulling to minimize the possibilitie of getting a knuckle. I work for the Illinois Central in East St. Louis, IL. We operate both UP coal trains to Paducah,KY. and BNSF coal trains to and from the Dynergy Power plant at Baldwin, IL.
It’s a great invention! You can do things with a dp train that are unheard of with a conventional set-up. It has some drawbacks though. It’s virtually impossible to maintain the signal while in long tunnels. All in all, much preferred to conventional operations.
Thesde particul locomotives use a product called “Locotrol”, I think locotrol 5 or 6. They have been using it there since the 1970s, all the equiptment is in the locomotive, both a radio reciever and a transmitter, the trainsmitter being in the lead loco and the reciever being in the DPU.
At one time there was a scheme which used the drawbar tension on the helpers, I think this was for mid-train units, to operate the throttles in those units.
Does anyone still use this scheme? I seem to recall it was one of the ways to overcome the issue of loosing radio contact in tunnels and cuts.