Another problem with solder joints is that you may want to change a piece of track or the track layout from time to time. You will need to unsolder the joints to get the track out. I agree with most others that it is much better and easier to just solder a small piece of wire across the joint. This is what we have done on the G&O garden railroad. I haven't bothered to solder any joints on my home layout. It has worked fine for the last 10 years. Both the G&O and my home layout use Atlas track.
NH Joe