I'm not sure what you are trying to do here. If you do what you say and create a folder called Retro and place it inside your Locations folder and have a track inside it - you have changed the track's location. In order for the game to find it you need to edit the .SCN file to look like this example -
SearchPath=.
Searchpath=Retro\
SearchPath=Retro\Autopolis\
SearchPath=Retro\Autopolis\Autopolis\
However, if you want to have an entry called Retro in your ingame track list you would do as Keith says and edit the .GDB file.
I suspect, however, that you want to have 2 versions of the track available - a normal one in its usual place and a tweeked one in Retro. This would be much more difficult and require a renaming of all files and folders so that the game sees it as a totally different track. If this is the case then you wouldn't need the first part - the Retro sub-folder. A track called AutopolisRetro could sit in the locations folder alongside Autopolis. If you don't get the renaming right there is always the chance of conflicts between the two tracks.
Good luck.