You lay the invisible track (preferably one visible in surveyor but not driver!) like normal track.
Then, down the bottom of the Track tab, there is an Advanced button. Click on it to bring down an extra meny with the Sp-line hieght tools. Use these by selecting the value and than clicking on the spline points (white/yellow circles) to raise the track, or use the Adjust Height tool to manualy raise and lower the track to any height.
Use one of the visible in surveyor tracks like his own track blue, red and green. It is best placed on the ground until you have the paths you want and any TM's, signs etc, then you just start raising the spline points to a level that want.
Captain.
Also be aware when you follow the above instuctions that the invisible track can have junctions, speed signs and signals (invisible best) just like ground track. You can have a complete planez layout.
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