That is either the ground wire (sometimes a wire) or the center rail pickup which connects to a pickup roller tab. Should be easy to solder back on. Maybe a tiny bit of solder flux, but not usually necessary since there is some solder already on the tab. I would recommend an inexpensive but excellent soldering station for trains like this one...
https://www.amazon.com/Weller-...QL70_&dpSrc=srch
Or more expensive and featured...
https://www.amazon.com/Hakko-F...QL70_&dpSrc=srch