I recently purchased a collection of new Marklin rolling stock from someone who sold them with the understanding that they needed cleaning. Apparently he had a layout near a kitchen area in his home and the stock has a slight but complete layer of dust and kitchen detris. Does anyone have any recommendations as to what cleaning solution should be used to clean the rolling stock? This presumes that each piece will need cleaning by hand with cotton swabs, not submerged in Fantastic or some other cleaner!

I would use warm water and dish washing liquid, doing the scrubbing with a soft toothbrush. A used toothbrush would be a good starting point , then wiping down with a dish cloth to get the remaining liquid off.

The water should be as warm as you can have it so that you can keep your hands in it comfortably.

If any items use transfers to add markings you may wish to identify these and be extra careful with them. Items that would fall into this category would be any wagons built from kits that Marklin did, and items like the vertical tank brown coal wagon.