The AAR (American association of Railroads), and internal trade organisation of USA rail management who actually do a lot of technical standards, came up with the AAR multiple unit system at an early stage to allow locos from different manufacturers to all work together. It's actually pretty clever, in particular the way it has been enhanced over time while retaining what computer designers would call "backward compatibility", so a modern all-electronic loco can be coupled to and control one from the 1950s with traditional electro-mechanical controls, or vice-versa.
Locotrol is a USA radio-based system for sending AAR control commands without wires to remote units in the formation. It had a lot of development, initially it was in its own separate unit, adapted from a "B Unit" (the non-driving ones) from 1950s USA streamliner loco sets, which of course had their own existing multiple unit connections to other units behind. Having overcome issues like transmission within tunnels, it's now as standard incorporated in new locos. It also does brake commands, which allows the brake pipe to be opened at points back down the train.