i also wanted to mod this nice layout.....
so i started with what wasn't allowed: adding tracks..... tried it with inserting spline... no response
i found a yellow spline and i could ad a track... it created a junction, but it looked ugly.... i went to quickdrive and had to save.... placed a loc on the track.... and the loc wobbled over the junction.... not nice but it worked...
i found a white spline and i could add a track... this time it was the smooth junction like i want to have.... and it turned into a yellow spline......
so i quickdrived and this was promissing a track with interactive industries.... i thought....
after saving and takin' a break.... and a sleep.... and a nightsift .... i tried to mod again...
but ... both junctions were disappeared.... and also the white spline..... i think it was even the only one....
this is very creepy... as if the software can detect and can heal....
so i tried the next one: adding a trackmark:

it worked fine and today it is still working........
daveric
PS (what is the functional or technical difference between a yellow and a white spline?)