The obvious way to arrange for lighting of each passenger rake or goods brake van is to install a function-only decoder and then either consist it with its locomotive or to address it as another train. Consisting is easy to do and undemanding when most passenger engines stay with their train for several journeys but, for goods trains, a brake van can be shunted off whilst a rake of wagons is marshalled and the engine might then move off elsewhere. I was pondering on how best to handle this and went through several options.
However, before I had to commit to any solution, a new MERG Pocket Money Project was announced for Automatic Coach Lighting. This responds to movement of the vehicle and switches the lights on, holding them for a couple of minutes after coming to a halt to allow for station stops. Unlike some commercial offerings, the MERG PMP detects optically, not by inertia, so the lights will stay off if the vehicle is moved whilst in a box.
There is in life a Law of Unexpected Consequences. In brief, it states that if you do something, it will lead to the necessity to make more changes than you planned. In this case, the addition of lighting to my coach fleet means that the interiors become visible. The consequence of that is that the interior detail needs to be good enough to be looked at and, in particular, no railway can expect to be composed entirely of ECS (empty coaching stock) workings. In other words, the interiors need to be populated ... which, in turn, can cause raised eyebrows if passengers are seen sitting in coaches in carriage sidings. The same goes for the locomotive and multiple-unit driving cabs where they are lit.