The North American remote radio control ("Distributed Power") was pioneered by an Ohio company called Locotrol about 50 years ago, whose success led to them being progressively bought out by larger companies, it's currently with Wabtec and factory-fitted to many mainline diesels there. The first receivers were in a separate, unpowered, former streamlined F-unit, often known as a Robot Car, with locos mu'd behind it using normal connections, and placed about two-thirds down the train. There was a lot of experimentation, different positioning, and broken couplers at first until they got it figured out. Keeping the communication going, through tunnels etc, is key. You only need one loco of each mid-train set to be fitted, it can then mu the others. It still seems to be developed, the latest approach seems to be to have two at the front, a single unit halfway down the train, and one on the rear. It is of course a separate nuisance to have to break apart a train to insert locos in the middle of it. All the issues of radio crosstalk with other trains, transmitting signals back to the front (eg fire warnings), etc have been long worked out.
With locos applying power from multiple points there are some wagons/couplers which are pulling, and others which are pushing, with a neutral point where the forces meet. This was a key issue, the neutral point would move up and down the train, particularly on undulating routes, and it was important that it did not pass behind the assisting mid-train loco. Not as easy as it sounds, and the source of a number of those broken couplers, which in any event are by no means unknown on conventional freight trains there. If I am not mistaken the mid-train locos have strain gauges on the couplers which participate in the logic of how the power is applied. Locotrol fitted units commonly have branding and a lightning-flash logo on the cabside indicating this.