There has been quite some discussion whether “common ground” is either not allowed or not recommended. This extends to many interfaces eg: one boosted section to another, digital to analogue, normal digital to programming digital etc, etc.

My specific question is: can I power LEDs for track detection using the wiring shown below without damaging the ECOS?



EDIT: Forgot to show a diode in series with the resistor, whis is required

In this case the connection is OK, since you use a separate trafo. In fact this is the setup Märklin recommends for level-crossings and other equipment where there is a ground connection to the track.

Of course this separate trafo can be smaller, use other voltage or even be a DC supply or a battery. There are two separate power circuits, and only one common ground.

The important thing to remember is to keep the power supply (trafo) for the digital unit away from every other part of the electric system. IN/OUT must be completely separate for the ECOS/Central Station/Mobile Station. This is obviously best acheived by only using the included connection plug.