Lovely work Dave
I use a small file on the underside of the rail to rough it up, then tin it with a drop of solder.
I then apply the wire, also tinned and simply melt the solder on both wire and rail.
Two seconds then hold in place for three seconds and done.
You should get a lovely, clean , shiny join (it does take practice though, my first few attempts had to end up in the bin )