Here is the full list of all 21 Stacking achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Completed the Lost Hobo King adventure.
Completed the Final Challenge.
Completed the Triple Decker Tank Engine adventure.
Completed the Gilded Steam Ship Adventure.
Completed the Zeppelin of Consequence Adventure.
Completed the Train Strike Adventure.
Collected all unique dolls in the Royal Train Station.
Found all of the solutions to all of the Royal Train Station challenges.
Completed all Hi-Jinks in the Royal Train Station.
Collected all unique dolls in the Gilded Steam Ship.
Found all solutions to all of the Gilded Steam Ship challenges.
Completed all Hi-Jinks in the Gilded Steam Ship.
Collected all unique dolls in the Zeppelin of Consequence.
Found all solutions to all of the Zeppelin of Consequence challenges.
Completed all Hi-Jinks in the Zeppelin of Consequence.
Collected all unique dolls in the Triple Decker Tank Engine.
Found all solutions to all of the Triple Decker Tank Engine challenges.
Completed all Hi-Jinks in the Triple Decker Tank Engine.
Found all unique dolls in the Lost Hobo King adventure.
Found all solutions to all of the Lost Hobo King challenges.
Completed all Hi-jinks in the Lost Hobo King adventure.