Now is where it gets tricky. Open an existing route in the route editor. Click on Window | Texture Select if it's not already open.

Click on Edit | Insert. This brings up a box (see below) with a bunch of terrtex files in it.  Mine always pops up on the right side of the screen, so you may have to grab the bar at the top and drag it over so you can see the whole thing. as you click on each file name a picture of it appears in the little window. Write down the name of the file you want to use for the default.

Exit all the way to desktop. Don't be lazy and go right to the extractor, this thing does a lousy job of clearing memory, so if you don't LEAVE THE LAUNCHER ALL THE WAY between each type of editor and each different route you open, you're playing with the kind of fire that will save information from one route into another, wrecking both of them.

Restart the MSTS Editor and Tools. Launch the Route Geometry Extractor.

The Route Geometry Extractor window opens. Click File | New Route

This displays the Create New Route dialog box. Type in all the stuff you want in the boxes.

In the Route Name field, type the new route’s name and description (such as “The Best Route of All”).   If you wish, you can click the Edit description button, type a route description, then click OK.

This name will appear in the list of routes when you run MSTS.

In the Directory name field, type a short name (such as “best01”).  To make Train Simulator routes more self-contained and easier to share with others, each route has its own directory of required files (including configuration files, terrain, shapes, textures, etc.). This directory name is the new route’s unique identifier, and the title of the route information folder that is generated when you create a new route.

Press "Ok". After a short time, MSTS RGE will tell you.....

Click File | Select route. (I assume you know to select the one you just created, even if the extractor is too dumb to automatically go there). This displays the "Select Route" dialog box, with a drop-down list box of available routes. In the Route list box, click the route name you just created, then click OK.

Click File | New Quad-Tree. The Route’s Directory name and “Normal Terrain” appear on the left of the status bar. Nothing else appears, since the quad-tree is still blank. The Quad-tree is the blank sheet on which the RGE tool lays out a grid of potential terrain tile positions.

HERE is where you insert what they left out and as near as I can tell from trying to add it or change it afterwards, you only get one shot at it.  Before "New Quad Tree" or after "Add All Selection Tiles" you might as well be trying to get milk from a male ostrich for all the effect it's gonna have.

Edit | Set Height Offset: Defaults to plus one meter if you don't change it here.

Right-click in the desired region on the map to display the context menu, then click Zoom Region.

Repeat this step to zoom in further. Continue this until "Zoom Region" is greyed out.

To zoom in further, click Zoom Window. Use Zoom Window as needed to zoom in to the desired degree. If you have a vector file for the region, you will see red lines indicating existing real-world rail routes. When the Zoom Window In is greyed out, you've gone as far as you can.

Drag a rectangle around the section of the map you want the route to occupy. IMPORTANT NOTE: Drag from top left to bottom right only or the next steps won't work.

Right-click inside the selected area, then click Add All Selection Tiles.

This creates an empty grid of black squares over the selected region. The smallest squares represent potential two-kilometer terrain tiles. You can repeat this selection procedure to increase the area covered by these grid squares.

To select the specific squares that you want to export as terrain tiles, zoom in as needed, then drag a rectangle around the region they occupy.

Right-click inside the selected region, then click Toggle Populated State.

The black squares you selected become blue. Remember that you may have to repeat this process a few times to get precisely the desired selection.  You can also select tiles one at a time by right-clicking a tile, then clicking Toggle Populated State. You can ensure that no region is selected by clicking anywhere on the map. The term “populated state” refers to whether you have selected a terrain tile to be created at the specified position. Whether you have selected a single tile or a region, clicking Toggle Populated State a second time deselects the tile or region.

Edit | Minimize quad-tree. When you see the message “This will remove empty Quad-tree nodes. Continue?” click Yes.

This reduces the route file to its bare essentials.

File | Save Quad-tree.
Edit | Generate Flagged Tiles.

This displays a message that indicates the number of tiles to be generated and the number of tiles flagged, then asks if you want to generate these tiles; click Yes.   In the status bar at the bottom of the screen you will see that the Route Geometry Extractor is creating the tiles one by one.

Right-click one of the blue tiles to show where you want the route to begin, then click Route Editor Start Tile.

Exit all the way to desktop.  Don't be lazy and go right to the extractor, this thing does a lousy job of clearing memory, so if you don't LEAVE THE LAUNCHER ALL THE WAY between each type of editor and each different route you open, you're playing with the kind of fire that will save information from one route into another, wrecking both of them.