how to restrict terrain generation to the inside of the track

I've created a track supposedly located on the waterfront, an illusion
more or less supported by the background image. Unfortunately, if I run
trackgen with the -a option, even when the "border margin" is 0 or a
negative value, trackgen adds the terrain to the outside of the track to
form that typical rectangle of terrain. This totally ruins the creative
intent and the appearance. The track without the terrain at all is
equally unsatisfactory, as if the track were floating in the air. 

I checked trackgen -h and tried the -B option, which removed the outside
terrain at some parts but not entirely (this example image
http://www.mojoimage.com/free-image-hosting-view-12.php?id=1295torcs-terrain1.png
 shows the problem area in yellow). Are there any other tricks to help
me get rid of the unwanted terrain so this track wouldn't look
absolutely ridiculous? Please don't point to the masterpiece also known
as Blender. In my desperation, I tried opening the -msh.ac file there
(which did open), but I literally can't do anything in that chaotic
environment. If there's any other option and maneuver left to try, I'd
gladly hear about it. 

st
http://stdesigns.orgfree.com/

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