Mannerock check the melting temp of your silver solder. It typically has a higher melting point than tin lead solder. The 63-37 tin lead solder is noticeably better than 60-40.
The cleanliness of your tool and the soldering surface is the most important factor.
A quick touch using the larger solder "gun", as mentioned by GRJ and others. is much better than trying a pencil type iron. You can buy extra tips for the solder gun and file/shape them to access smaller areas. Reducing the mass at the end of the gun tip by filing does shorten the life of the tip. Likewise the high powered solder stations usually have multiple shapes of tips to choose from.
I like to pretin the wire first and have it reasonably hot before taking it to the rail solder area.
Also you can put electrical alligator clips, or clamp crushed aluminum foil onto the rails near your solder area to get the heat away from the rails and protect the plastic ties.
