There may be special circumstances where soldering a wire to GarGraves track is required but for most connections there is no need to actually solder the wire to the rail. The following pictures illustrate a method to power GG track that I learned from Mike Reagan back when he had the original Train America store going.
First strip a little more than 1 inch or so of insulation from the wire you are using. I think the wire in this picture is 18 gauge. Then apply a small amount of flux to the wire and heat it with your soldering iron. When the flux has evaporated, touch the solder to the heated wire. Once the solder has cooled, gently bend the soldered wire in a slight zigzag pattern. /\/\/\/\ The wire in the picture came from my layout when it was dismantled so the soldered part is rather straight.
This photo just shows the end of a section of GG track. What is important in it is the opening in the rails that is just above the ties. The point will become clear in the next picture.
Insert the soldered wires into the openings. If needed, remove some of the zigzag. The wire should fix snugly but you should not have to force the wire into the opening. Do Not put the wires in the top openings, that is where the track pins go. I always used black wire for the black center rail.
This next picture was intended to show the wires fully inserted into the rails and then bent down but it isn't really clear. When the wires are bent down I can't remember any problem getting the adjoining track section to fit tight.
I wired the entire layout in the next picture using the method described above and did not have any problems with electrical connectivity. I usually put a power feed every other full length of GG track where possible.
This last picture just shows a couple of power feeds going through the layouts plywood/homosote surface.
I hope this helps anyone having difficulty soldering wires to GG track.