Path Finding On Certain Path?

I am trying to develop a basic roller coaster tycoon game. My problem is that i'm trying to come up with a solution to allow my NPC's to follow the paths I build but while staying on the path instead of slicing corners and walking through the grass.... I was thinking A* but idk if that's a good way. Right now i'm just using roblox's built in path finding but there is no way to ignore certain parts. Any Ideas?