If the sets of locomotives are broken up, with some on the front, and some in the middle or on the end (or all of these together) then radio control is what's being used. This was originally developed many years ago, and a couple brands existed -- Locotrol was the most common. You could either mount the receiver unit in a boxcar, which would be coupled to the mid-train or end of train locomotives, and give the MU signals to them, or else you could mount the equipment directly on the locomotive. If I recall correctly, you had "transmitter" and "receiver" sets -- none of these old ones, thus, could both send and receive.
When the units are in a set, the various electrical signals needed to control trailing units are sent through a "multiple unit cable," generally known as an MU cable. This cable has numerous wires inside of it, each of which carries a specific required signal. It's also used to sound the alarm on the lead unit if a trailing unit experiences trouble.
Of course, you most often find (at least, found, in the old days) that trains with mid-train remote power (like the old Detroit Edison unit trains that ran over Penn Central where I lived) had a multiple unit set on the front, and a multiple unit set in the middle or 2/3 of the way through the train. The lead engine was controlling the trailing units up front via the MU cables. It was also radioing a signal to the receiver unit in the midtrain set, which then controlled the other unit coupled to it via MU cables.
Braking was a different matter; the mid-train or end-train units' air brakes were tied into the automatic, or train brakes, in the usual way, just like cars in the train, and did not operate like the front end locomotive set's independent brakes.