*.cuh is the format for carsets / mods in CSM. it does usually not contain tracks.
*.tuh is the format for tracks in TSM. it can be opened and extracted with tsm.
*.tup is the format for trackpacks (more than 1 track) in TSM.
a track consists always of a dat- and a wad-file. to create a tuh-file, use the export function in TSM.
some mods refer to a specific track set (f. e. the 2020 mod will look for a trackset called "2020.ini", the 2019 mod will look for a trackset called "2019a" and "2019b" and so on), even though they do not contain tracks.
you will find many tracks here or on gp4forever and can create your own tracksets in TSM.
If you don't want to use CSM / TSM, you can also load your tracks with GpXPatch
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