I've soldered to the "Alien metal" of the Atlas switch, I just scrape all the blackening off it. I always thought it was the same as the rails. We had a bunch of older Atlas switches that the flimsy thread burned out and we had to jumper. Since I didn't want to lift them, I just run a wire under the table between the two points. I use a heavy enough wire so I only have to do it once.
The black finish on the center rails of our Gargraves track and Ross switches is also very hard to solder. We did something similar to what gunnerjohn did. I use a small narrow abrasive bit in my Dremel. I put the rounded tip on the side of the rail (flat side area between top and bottom of rail) and then grind off a quarter inch or so of the black finish. Then it is easy to solder.