Many coasters may have custom scenery attached to it, or used CTRs. If you don't have these corresponding custom things, the coasters will crash, so this might be the cause here. Do you know if the coasters you downloaded required any custom content?
All we need is somebody to lean on.
If the coaster track file was not built using one of the standard ingame coasters that your version of the game has then you need to get the custom coaster files and install it and all of its' track pieces into the game.
If you had downloaded one of my coaster track files but you don't have RCT3 Wild! that has the extended coaster then my track files wouldn't work for you.