Has anyone experience with using isolated solid wire instead of the usual colour-schemed multicore wire
as supplied by Märklin and Viessmann and others?
On my next update I would like to try this. I realize the hazards of shorting if the isolating lacker gets a crack by rubbing against a sharp metal corner or likewise, also due to not being flexible it can't be used where the decoder needs to be located in the slepptender, nor between cars and coatches.
The advantage as I see it, solid wire is easily shaped and has a smaller overall diameter.
The picture shows a strip of Viessmann wire and a strip from a secondary coil of a 240V to low voltage output trafo. As for current conducting capability, the solid trafo wire has a larger diameter which I think fully compensates for the smaller diameter multicore wire, although the metal surface of the latter is larger).

Your probably aware of this already but you need to use more care when preparing varnished wire for soldering. The varnish must be thoroughly cleaned off at the solder location in order to make a good bond.

regards,
George