I have not experience any problems by using an index or business card between the rail ends. I solder the railjoiner solid to one side of the joint and run a small (#28-30) jumper wire around the gap. I solder a feeder to the bottom of the joiner at every other joint (feeds 6' of track at a time. I try to stagger my joints in the curves. I have few derailments except for operator error. I also file a bevel on each rail end as was mentioned earlier. I use the same gap winter or summer, but my train room doesn't vary over 25 degrees.
Sorry, I did not see the insert on this new style of page. I guess I need to post a complaint!