The MSTS 1 Route Editor can be a beast to use, but it's capable of some amazing results, here you can talk with the wizards that are building some of the fantastic routes available and learn how to make your own.
I'm wondering if this is NOT a video card issue - the image looks to me as if the "V" key has been used to punch a hole in the terrain (like building tunnels) to make sure that the turntable has a pit (otherwise terrain fills the turntable up to track level).
You may need to add transfers carefully around the turntable are to fill in the gaps in the terrain.