So, yes, I am soldering the rail joiners, and the gaps are not very wide, just wide enough to make me obsess over them. About 0.080", or 2/25". Even my pickiest of cars (which will soon be destined for extreme aging and abandonment on a forgotten spur - that car really ticks me off some times) doesn't have any trouble going over the gaps, they're just loud and big enough to bug me.
I have been using flex track for the part of my layout that I am working on right now, and I thought I was very careful with my cuts. When I test fit everything together, it all looked good and tight. After soldering, I noticed several gaps in the rails. Fortunately the sections are straight. It just so happens that the are right after a turn out and just before a lift out section, so there are several gaps within a 4 inch section of track.
One of the rail joiners shifted on me as I soldered it, so I might have to re-do that one, but with gaps of about 0.080", you think it will be fine? (just loud.)