Flux, flux, flux. Use a fiber brush to clean the solder spot on the silver rails. Use a file or whatever is available to remove the black from the blackened rail. Flux the rail and put a drop of solder where the wire is to go. Flux the wire and add solder to the exposed end. Place the wire over the soldered portion of the rail and apply the heat. It’s that easy to get a solid joint. It would always help if the poster would advise the type of soldering iron used as well as what type of solder will be used. Good luck.
