Mini Metro is a subway themed puzzle game. Not a true simulator, although it does have elements of simulation in it.
A very simple game that lets you focus entirely on the puzzle without anything you don't need to pay attention to, the goal is to connect the stations and get the people to go to a station that matches their shape in an ever expanding city which just keeps getting bigger and bigger until you can no longer handle the influx of people.
There are three basic shapes- The square, triangle and circle, and you begin your game having to ferry passengers who each are one of these three basic shapes to a station with the same shape. Square passengers to square stations, triangles to triangle stations, and circles to circle stations. You then get more stations to connect, you have water you need to tunnel under with limited bridges, and eventually you will get unique station shapes to connect up to your lines.
While any triangle station is good enough for a triangle passenger, a plus sign passenger will only go to the plus sign station. This means that you will need to keep your lines connected to each other as well as to the three basic stations, and that you will need to figure out where and when to transfer passengers.
Losing a game happens when the waiting line at a station is too long, which will slowly fill in a circle around it to show you how close it is to being a game over.
Passenger AI is rather simple, but it doesn't need to be complex either. They go to their nearest matching station, using as few lines as possible. An trains pick up passengers as long as they get them any closer at all, even just a single station.
Of note is that when a train goes both ways, passengers will not get in on the one that heads away from their destination, instead waiting for a train that heads the right direction.
And once you're done with most of this, there is extreme and endless mode. Endless mode is a game without losing, so you can relax and build your subway system without worrying about losing. Extreme mode is similar to the original game, but where you can move and remove tracks and trains at will normally, they are locked into place in extreme mode.
On losing a game you can continue playing in endless mode.
In the end, it is a simple but worthwhile game to play. It doesn't have a lot of special mechanics, flashy explosions or incredibly detailed environments, but all it does have works towards the same goal - An enjoyable puzzle game you're sure to get plenty of hours out of.