I just got this one. Seen a few posts and lists regarding this. I ended up going through each park, sold ALL coasters in the park, them built the coasters that have a whoa piece in it. I just built the smallest loop and used the special whoa piece for each coaster. It unlocked in the 3rd park, so I must have missed using the whoa piece somewhere.
Also noticed that I got a message saying I had researched one of the coasters that have a whoa piece. If you are missing a coaster from the various parks, you may have some research to finish. You should see 4 whoa coaster in each park and 5 in the last one.