A RR track is a straight, connected to a curve, connected to another straight ... even an "S" curve has a short segment of straight track connecting the 2 curves, as do all turnouts, have a straight track lead in.
A straight track, leads into a subtle curve easement, then enters the curve itself ... then exits a subtle curve easement, and then on to another straight.
Laying track curves in Trainz is really easy when the curve is short, and under 45 degrees ... when a curve makes a 90 or 180, the curve is very distorted, and needs many additional track spline points.
A curve usually has two end spline points, and 2 more controlling spline points in the middle of the curve ... some curves require 5-6-8 or more controlling spline points in the curve section.
I use this method, using the Trainz ruler ... and make 2 intersecting straight tracks ... measure back an equal amount from the intersection on each straight track (@ 100 - 500m) ... place 2 track spline points at the ends of the 2 rulers ... connect those two distant track spline points with a piece of track ... delete the un-needed intersecting tracks
http://www.youtube.com/watch?v=YIZCvdB0Vn8
http://www.youtube.com/watch?v=b3HD6lZC3XA
Another method is by using FT assembled track sections
Which you simply slide into place, use it as a tracing guide, for laying single track on top of it
By adding four 10m FT Straight on to the end of an an assembled curve, you get a subtle lead in / lead out curve easement