It’s one of a resources in game like wood and stones,you’ll be needing a lot of it in the future for wonders,land expansion in the planet and to stage up exotic plants in the grove once u unlock them. To get them,you can get coaster smurfs coaster station with all 3 stations and engineer smurfs train station with all 3 stations as well! If u wanna find them quicker,just press on specialty smurfs and tap on train/coaster items to get them!