Add on Led's need resistors to ensure not blowing, and inhibit flickering. It isn't rocket science. I use 2.2k ohm 1/2 watt. I thought they might be too strong, but they worked.
I had some nano Led's that the built in tiny resistor was too hot. These resolved it. Marklin LED kits have them built in.
Be careful when tapping into a locomotive motor or decoder for current conducting couplers. Research the new items from Marklin with this feature. They warn of possible shorting out if the car is of a metal bottom, or bogie with a metal coupler. You may NOT have two cars in a train consist with more than one pick-up shoe!!!
New Loco's with these use their decoder to turn the coupler current on and off. You can exchange the current coupler for std. ones if the trains is like this below;
You also need to "NOT" have one installed betwween the Locomotive and train if it has a cab control car with a decoder for lighting other cars.
The new current conducting couplers only couple to each other, not to standard non conducting ones.
You wil no doubt get more or better advice from others.