Symmetrain is a casual family-friendly game for iOS. It is a vertical auto-scrolling spot-the-difference challenge. The game features a variety of landscapes with different themes (e.g. grass countryside with huge mushrooms). The player can choose between several trains, some of them slow, others fast.
The goal of the game is spot as many differences in the passing landscape. Tapping the right objects will reveal the missing counterpart while tapping the wrong places will slow down the train. The game ends when the player has made more than the allowed number of errors.
You can get the game in the iOS App Store here: https://itunes.apple.com/app/symmetrain/id725274537