Hi David
This is a well-known and well-documented problem. The current in the digital world is not a "pure" DC current since it is "interrupted" by the digital signals which have to be communicated to the decoders via a reversal of polarity. So whereas there is mostly +20V (approx.) on your rails, every now and then it's -20 when a digital signal comes along. This happens many times per second, and the visible sign is the flicker that you see on your second coach.

The reason why your first coach is OK is because you're using the orange wire instead of ground (the rails) directly. Now, the orange wire supplies a straight, smooth +20V all the time, so there is no flicker in that coach.

So the solution is to somehow get orange in the other coaches as well. Some common ways are:

1. Use two-pole current-conducting couplers that can carry both the brown/red and the orange.
2. Fit sliders to the remaining coaches and "make your own orange" by connecting slider and ground via 2 diodes (disadvantage: you're back to a slider per coach).
3. Use a diode and a capacitor between ground and brown/red in each coach; the capacitor will "smoothen out" the digital signal.

Hi. Thanks for info. I did use two pole couplers but change to the single pole which is better than Viessmann which are not always reliable.
Marklin in their instructions with the decoder appear to indicate that using their single pole coupler other coaches can be added..they say (First coach would use its existing ground as the ground return ) This does not make much sense to me.

What would happen if only the red and brown were connected to both the pickup and ground and then to the lighting bar...leaving out the brown/red and orange. F1 would no longer work but would the lighting would be turned on and off just by selecting the decoder address.
I am sure I have had this working in the past without any further connections but may be wrong.

David

Many thanks Svein.
My problem was that Marklin indicated that it was possible just using their decoder but it is clear that this is not possible and I will now use suggestions as above.
Thanks for assistance guys

David

Correct, as the LED uses only one half of the waveform, the upper or the lower, depending on how it is connected. My suggestion is to solve this with a small bridge rectifier in each coach.