I clean the rail as much as possible until it is "shiny" I cut a small piece of thin brass into a strip as wide as the bottom of the rail. I then set up the pieces to be joined on top of an old bathroom tile. Heat with gas torch & add solder. After the solder runs, let it cool naturally, dont use water to cool qiuckly as it can alter the strength of the joint. When joint is back to room temperature I then wash the joint with cold water to clean up any residue.
The small piece of brass on the bottom gives great strength to the rail. I use 60/40 electrical solder. I was using Bakers soldering fluid but found it was eventually leaving a stain around the joint and any wiring would fail in that area.
I use this method to join up lengths of rail that is too short for a good run by themselves, but joined together can make a great straight siding. With the cost of rail, we tend to waste nothing! It can be a hastle slipping the ties onto the rails but a bit of manipulation will acheive results. Or make your hand made ties for a "bush" siding and hide ties under some covering (dirt or concrete). Remember that ties can perish with time but rails will be around as long as you.
Hope this can be a help to someone.
Regards from Andrew, Sandbar & Mudcrab Railway