Dremel with a wire wheel to clean the rail, use flux and a hot iron. Its pretty easy to solder to Ross track, but you need to get the oil off of it first.
The easiest thing to do is flip it over, stick a flat head screwdriver in the seam of the rail, stick the wire in the gap and solder it, but if its already put down, its too late for that.
I definitely agree with the split and solder approach.
Some folks don't even bother with the solder. for tubular track like this they crimp a spade lug onto the end of the wire, split the bottom of the rail open just enough to jam the spade into the bottom of the rail. It's not as good, but it's certainly functional.
For track that's already down you can solder to the sides of the rail, but again it needs cleaned first. Then pick a spot on the outside of the outer rail so that the flanges aren't affected by your wire, and for the center rail just stay as far down as you can.
Remember that Ross and Gargraves track have wooden ties so the two outer rails are not electrically connected. If you have an insulted track section somewhere you need to make sure you don't end up with a dead rail section. It is common to jumper the outside rails together to improve the electrical connection to the train.
Good luck