Could you take a few screenshots by pressing K in the simulator then upload said screenshots to this thread using the "full editor" button on your reply?
The track will bend and bank on its own, whether you use roll nodes or not. To keep it flat, you need to place roll nodes along your track and set them to zero. As mkingy said, posting pictures of your problem can greatly help us to understand your problem. Take a screen shot, then use the "upload attachment" tab, which is part of the full editor when you reply here on the forums.