We've been seeing this issue with splines and track. One of the things that helps is to ensure that your screen resolution and refresh is the same in game as is for your monitor. The other thing that helps is to precache your content. This usually occurs when downloading content and installing but if there's been a lot of content installed, this takes a bit longer. You've probably noticed the precaching message at the top of the loading screen as well which is exactly what's going on.
To ensure the content is precached, do the following:
At the Launcher, click on Trainz settings.
Click on Dev tab
Check the box for Advanced Developer tools.
Close the Trainz settings.
What this does is enable the Trainz util run box on the Launcher.
Back at the Launcher, click on Developer.
Click on TrainzUtil.
This will open up a small run box.
In the box, type in prebuild
Depending upon how much content you have, you may want to run this overnight if you have a lot because this process takes a while.
John
Trainz User Since: 12-2003
Trainz User ID: 124863
Trainz-PLUS: 117669