Copper will be a better electronic wire than the silvery/chrome stuff for two reasons: first, it's COPPER, and second, it's more robust and you'll have less voltage loss over distance if you have to have one or more feeders in excess of 3'.
Note that I used the relative term 'better' and did not quantify the difference because I don't know where I'd find it, don't know what the 'silver' metal is, and I can't give you specifics otherwise. But that #20 should be quite good, if a little thick maybe. Hammer the tips flat before soldering so that you don't interfere with the wheel flanges. Or, as you do this, hammering them flat might be pointless if the Code is 100.
In general larger wire is "better" if you already own the wire.
Something to do: get a multimeter and measure the resistance of a good length of that 'silver core' wire, to compare it against 'table' values of 24ga (and 20ga) copper. You may have something of low resistance for future work. People here can advise you how to do the measurement precisely enough.
Thanks for your comments! I will use the #20 solid brass core.
Another quick question. As I get ready to solder the feeder wires to the track, I planned on cleaning the spot where the wire will attach with a wire brush. I have two wire brushes that look like pencils. You twist the top to extend the brush as the wires wear down. One is brass and one is steel. I am using Atlas, code 100, flex track. Which will be the best brush to use, the brass or the steel?
Jack
Brass is good enough. You just want to remove any paint weathering and minor oxidation.
There are fiberglass brushes similar to your description. There have been write ups using those to remove lettering. They might be too tame to remove spray paint track weathering efficiently.
Henry
COB Potomac & Northern
Shenandoah Valley
Henry, thanks for your reply. I think I am all set to start adding feeder wires tomorrow.