There is some excellent advice in fsm1000’s post, above. (The others are most helpful, but I just wanted to add my support to the thinking in this particular post for the sake of comment…)
Even though flextrack, at least the Atlas Code 100 with which I am most familiar, is meant to curve, and to be joined on curves, it isn’t really fussy about it. In other words, it would prefer to remain straight…if you don’t mind. And that is where many of our troubles lie, in the laying and later in the running. If you don’t take your time (very important words there…) and get your roadbed placed well and groomed well, if you don’t get your rail ends aligned well, and fastened in such a way that they won’t spread out towards straight again due to tension, and ensure that both rails are very nearly in the same aspect relative to the axis of travel, you will be one unhappy fella.
I took the tip, I believe it was from Chuck Tomikawa, to use something that will remove metal safely and bevel the flange face and the top face of every single rail end, bar none, and for gosh sake do it on your faster curves on the main.
Avoid gaps longer than 1/8" like the plague. One sixteenth is far better. If you somehow end up with a gap between rails, especially when everything else about that section is sooooo great, consider filling it with carved styrene, epoxy, or solder. You could also just nip a few inches of track from that end and cut a better fitting one to insert in the gap.
Get down to eye-level like a four year-old boy, and cast a sober, critical eye on your rails. Do they curve nicely, or are kinks apparent from that viewing angle…kinks that are vertical in nature and/or horizontal? Your cars and locos will remind you of that area each time they pass, regardless of the direction.
Absolutely, yes, test often with a powered loco and your fussiest rolling stock. Don’t expect that any problems they encounter will be mitigated by the mere passage of time. I