Probably there are corrupted objects in your route or scenario.
Finding them might not be easy.
They might be assets, or some element particular to your route or scenario (tile, terrain, decal...).
The first thing to do is a verify of local files in the Steam client.
This will repair your assets (this will also reset any mod applied to Steam DLC, so make back-ups of that).
Do it until there are no more files to repair.
See if that fixes your problem. If not, you'll have to find the culprit manually. It may be a tile, a piece of scenery, a bad instruction in the scenario, etc.
The only fail-proof prevention method is versioning, i.e. saving your work to a different version at every save.
This way, when you get this kind of problem, you can roll back to an earlier version until the problem disappears.
See
this thread for an example on how I do it painlessly using a script.