Issues with slopes and corners:
Problems: At speeds notably higher than the default speed, minecrafts going to turning, or going uphill can get derailed, or bounce back and change direction.
Reason: The issues, where the cart gets derailed at turns, or bounces back from upward slopes, is something caused by the vanilla minecraft's engine. If the cart goes too fast to a turn, it gets derailed (like in real life (I don't have any idea why notch did it, as normally they cant go that fast)). With the slopes, I think the cart goes through the tracks, and hits the block behind, the snaps back to the tracks, and starts going down.
Fixes: I am not planning on fixing this. It might be possible, but it would require checking every minecarts position with small intervalls. and looking if there are tracks/turns/slopes ahead. The cart would have to be manually moved to the desired location with really small steps. That is something which would really make a server LAG. It would also be really hard to make it work smoothly.
Workaround: Slow the cart's speed down with speed setting signs before turning, or going up a slope.