One aspect of this topic and extremely relevant to @gunrunnerjohn's excellent diagram of the AIU connector 12V pinout is that many folks, myself included, using relays have run into the power problem when you energize an entire bank of relays. As an example, at our club, we use several time delay relay modules for accessories in our amusement park area. Because we powered those timer relay modules off of our accessory power bus, there is a small 12V switching regulator module. We kept adding more buttons, more relays, and visitors would just go down the line of 12 buttons and press them all on, and so the current of holding that many relays in the active state would kick out this regulator on thermal. The solution was a bigger regulator, but also break up the multiple banks of relay timers, and put them on individual regulators.
Well, where this story has a parallel to typical train usage cases, you might wiring wise make sense to put the AIU nearest the accessories being controlled rather than a whole bunch of wire some distance from each accessory to the AIU. The problem is, then this thin AIU cable wiring is carrying that 12V from the TIU to your TIU, and depending on how many relays are active at any given time, the amount of current the AIU draws, and more importantly,the voltage drop across the distance of the cable. So, on long cable runs, it might be highly desirable to provide a local 12V regulated DC source at the AIU, especially one when a lot of the relays spend much of the time activated all at the same time.
And let's be honest- even Lionel ran into a similar issue with the LCS cabling system and a single power source. That's why only through tech support, you can order the special PDI cable PCB that allows you to inject a local source into that system. Many LCS modules are just banks of relays and the more relays active at a given time and longer cable distance, the worse the problem becomes. https://ogrforum.ogaugerr.com/...itations-through-pdi
