The procedure to add track polygons has been changed since previous versions, and is now slightly less flexible but is now almost bug-free (now it should work!):
Switch to high-resolution view and point mode; select the Split/Merge tool.
Split the vertices around the polygons you want to duplicate.
Switch to polygon mode; select the polygon you want to duplicate, and use the Edit-Tools Duplicate to split it into as many pieces as necessary. In order to avoid bugs, it is no longer possible to move a polygon from a track node (a "row") to another one.
Switch back to point mode; move and merge the vertices to fit the intended disposition.
Switch back to polygon mode; set the passable/non-passable properties for each newly created polygon, if they differ from those of the originally duplicated polygon; similarly for the textures. In the case of a High Stakes track, don’t forget to properly check the "no forward/backward neighbor" buttons in the Polygon Properties dialog box if necessary.
Switch to half-resolution view, and modify its contents if necessary (polygon deletion and duplication work like in versions up to 0.05).
Repeat the previous step in low-resolution view.
NOTE: the structure of the track polygons imposed by the NFS game engine is extremely strict. You should always ensure the following:
each of the 8 track nodes within each track block should correspond to a single row of polygons, ordered from left to right and adjacent to each other along a common edge. Starting with v0.06, T3ED ensures this automatically in most cases, but you should be careful when importing tracks edited with older versions.
each track polygon should be facing upwards and with the correct orientation. Once again, this is automatic in version 0.06 and above, but you should be careful when importing tracks edited with older versions.
the track polygon type (Polygon Flags dialog box) should be set to 0 for non-passable polygons lying in the middle of the track, and to 14 instead for non-passable polygons lying to the side of the track; the polygons with type not equal to 14 should form a single row for each track node.
each passable polygon should have the collision detection flags set correctly; in NFSHS the absence of passable neighbor also has to be indicated (in the Polygon Flags dialog box for forward and backward neighbors; automatically handled for left and right neighbors).
the road width should be adjusted in order to avoid invisible walls.
If any of the above fails, the track is likely to contain invisible walls and other bugs; remember, adding track polygons is a complicated process, so be sure to check carefully what you do.
A final note: if you want to create a track which branches into two separate ways, it is still necessary to keep track polygons between the two parts of the track (remember each track node should be a single row of adjacent polygons); you can either give them the proper textures, or even in some cases make them transparent (select a proper empty texture).