When you place assets in the route editor, they will be assigned a 'detail level' value by the game. This value determines whether the asset will be shown in game, or not, depending on the users in game graphics settings.
Assets with a detail level of '10' will always be shown, even if the user is playing using the lowest graphics setting. Assets with a detail level of '1' will only be shown for users with their game settings on 'highest'. The game will randomly assign each new asset placed a value based on an allowed range in the assets blueprint. This is to theoretically reduce the graphic cards work load on weaker computers.
To check or change an assets detail level, hover your mouse over it to highlight the asset, then double click on it. This will bring up a flyout menu on the top right of your screen. The detail level will be shown in an editable text field. You can click on it, then change the value. (see below)
Detail Level 1.jpg
I suggest making all important assets have a value of 10
You can quickly check what detail level assets have been assigned, by moving the scroll bar at the bottom right flyout. (see below)
By default, when you open the route editor, the slider, and its associated detail level will be set to match your current graphics settings for Train Simulator.
Detail Level 2.jpg
By dragging the orange slider button left and right, you can quickly see what assets are set to what detail levels.
Detail Level 3.jpg
Detail level 4.jpg
As you can see above, DTG made the detail level slider display assets with values INVERSE to its setting. (just to make things clear as mud)
Slider set at 10 - Assets with all values appear
Slider set at 9 - Assets with a value of 2 or higher will appear
Slider set at 8 - Assets with a value of 3 or higher will appear
Slider set at 7 - Assets with a value of 4 or higher will appear
Slider set at 6 - Assets with a value of 5 or higher will appear
Slider set at 5 - Assets with a value of 6 or higher will appear
Slider set at 4 - Assets with a value of 7 or higher will appear
Slider set at 3 - Assets with a value of 8 or higher will appear
Slider set at 2 - Assets with a value of 9 or higher will appear
Slider set at 1 - Assets with a value of 10 or higher will appear
To summarize: Make assets that you always want to appear have a detail level value of 10. You can allow assets that are not as crucial have lower values.
If you are placing lots of repeat assets (like trackside grass), I suggest placing one, setting its detail level, then using the 'copy' and 'paste' functions to quickly paste more of that asset with the same detail level settings.