^ What BNSFdude said.
Some ground textures have a setting to have 3d grass/flora appear on them. The grass generally only appears close to the users location, and will disappear as their train, or camera position moves away.
Each route will have a 'sheet' of all of the different procedural flora types. You can change the type and density for each by editing the texturing.xml file found in the route providers 'Environment' asset folder.
Procedural.jpg
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