@Paul Kallus Paul as far as I understand the rail joiners are meant to conduct electricity. Most of my Atlas loop runs perfectly fine using only the joiners plus joiner wire feeds shown previously. I have drops wherever there are switches. Part of the (my) issue is using "used" joiners which may fit rather sloppy. Put them on a roadbed that flexes a bit and the movement causes the electrical connection to open randomly. (I'll fix the roadbed issue later.) But with a large layout, I would be inclined to solder all the joints so there will be one less thing to check.
Good question on cleaning the inside of the joiners especially the blackened ones. Maybe it does not matter as long as there is a solder connection between the rails
