To solder the rails you will need a solder with acid. But even then this is NOT recommended. You risk melting the plastic parts, and the acid rosin core solder can rust your rails.

Better to stay with the railjoiners, or make small shoes out of spare joiners, that you then clamp on the isolated track-piece. (I think Dale Schultz uses this method, and he will probably tell you exactly how).
Another alternative is to use a stiff thick wire that you squeece a bit, and then insert it between the rail and the plastic trackbed.

Hi again,

Thank you very much for your help. Very usefull tip.
Dale I will keep in contact with your site.

Best regards,