You can get a good curve in spite of the jittery markers as the spline track will naturally curve smoothly. Just select a few points of your marker curve to make sure you track hits the center of those and you'll get a good curve. If you mean accuracy then the best way to go is using RWDecal
http://www.rwdecal.co.uk/index.php RWDecal is a great program and a great timesaver in many ways as you'll learn uses for it not advertised by the author. It's very easy to use. Here's a quick snapshot of an area with RWDecal applied.
rwdecal.jpg
I'm not in an area where the track curves. But you just lay the track right on over the decal and you just can't get a better solution. Afterwards the decal is great for placing things. Railworks is great for the multiple marker files you can setup. RWDecal coupled with that advantage, helps a lot.
Rich
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