This award winning app, apparently created by a pair of German students, is a delight-- it requires a different skill set from most of the 'solitaire' games available for iOS, has marvelous visuals (more on that in a moment), and is simply fun to play. The home screen comes complete with hidden "Easter eggs", so that as you tap on certain of the passengers in the train station you get the design and music credits (among others), and even a comment from the Blue Bird of Happiness, as well as a sound track of a station, with overhead announcements.
If I have any complaint at all, it is that the trains run too fast-- it's impossible to take the time to actually admire all that has been included in the (too often!) asymmetrical scenery through which the various trains pass.