With beta24 we've added the first lot of achievements into Mini Metro. All cities past New York City have been locked off, with score goals to hit in other cities to unlock them. These first achievements aren't designed to be particularly difficult, especially to seasoned players, more to gently introduce the cities to new players. More involved achievements (including much more difficult conditions to unlock extreme mode for each city) will be coming soon.
Beta24 ended up on Steam before we made a post about beta23! The big new feature in beta23 is the statistics graph. You can find it from the game over screen after you end a normal or extreme game, or reach the city limits in an endless game. Finally, the greyed-out Statistics button does something!