How hard is it to retro fit a Decoder to the playmobil trains. I've heard of people doing just never seen the wiring.
It's not very difficult. In Playmobil locos there is enough space for the small H0-decoders (for 1 motor) or the bigger G-decoders (for 2 motors like the 4052).
4 soldered connections to drive. 2 for one light (1 resistor). 2 for (easy) smoke with Seuthe/LGB (with USA Trains + fan + Zimo Decoder it's another thing).
In the Klickywelt there are a few boys with fixed layouts (inside or in the garden) which switched to digital cause of the opportunities. Independent driving of locos with lights (front, back, cab, suspension), sounds, smoke, electric couplers,...
I have a Roco Multimaus (Multimaus alone: power for 1 to 3 Playmobil motors) and a booster (power for more than 10 motors) and each loco needs a decoder (digital controller) with 1 Ampère per motor (or more).