Do NOT Solder the Fishplates as you have suggested in your initial post. If your track is new, the connections should be very good. If it is not new, you can improve the connection by nipping the Fishplate ever so slightly with some pliers. (The emphasis here though is the EVER SO SLIGHTLY).
The sliding Fishplates allow the small amount of movement caused by Temperature changes. Your track may warp if the joint is soldered.