I have the problem that when I fly away from the starting station, after a short time the ground is simply no longer visible. View from a little further away: View from the start station
I tried it and the floor isn't there either. If you get out and walk off the tracks, you will fall into the infinite depths.
This isn't a landscape but instead it's just a giant plane, which by far isn't a proper way to do things. I think you will have to start again and actually generate the landscape tiles just like in the training stream. I think using a giant plane for building a route is a pretty common misconception as DTG did this on their very first editor stream where they were showcasing various basic concepts.