Mike, what material did you use to fill the gap in the rails?
Using 60 40 solder, I hold the copper wire in place and apply heat with an old Weller D550 soldering gun, which has two heat setting 240/325watt. A standard Weller gun 140/100 watt would also work fine.
The solid copper wire, (I've used 18 ga to 12 ga), is bent "L"-shaped and held in place in the small open spot, bottom of the rail, as solder is applied.
Rail was cleaned with a small screw driver before apply-ing solder. Note: the blackened material has to be removed from the center rail, but, excessive scrapping, and cleaning, also removes the small coating of galvanize that is important to a good solder connection.