Rather than spend time trying to clean the joints, which will not work well enough unless you take apart the track, solder jumpers accross the rail
joints. It sounds like alot of work, and it is. But the long term results are more than well worth the effort.
Drill a 1/8" hole into the side of the rail at each joint. Keep the holes about 3/4" to 1" beyond the joint. Clean the area around the holes with a
Dremel or your tool of choice. Take a piece of 14 gauge stranded wire, insulation stripped off it's entire length, and poke each end of the wire
through the holes you just drilled. Now using a butane mini torch, heat the rail and the wire and touch the heated area with rosin core solder. When
you see the solder being drawn into the hole you know it has made a good bond with the brass rail. The stranded wire will heat up much faster than
the brass rail, so keep more heat on the rail with the torch. You may get some melting of the plastic tie, but if your track is ballasted you'll
never notice it. You can use a 300 watt soldering gun, but it takes much longer to heat the rail.
All of the rail joints on my own railway are soldered with jumpers in this manner. I have never regretted the work it took.