Mini Metro, currently available in Early Access (the game can be purchased but is still in development so some elements, such as sound in this case, are missing or unfinished), is a game about moving people. If this seems like an oversimplification, I can tell you with confidence that it is not. Where other games of this sort focus on such elements as budget and specific regions, population density and specific destinations of interest to that population, Mini Metro instead focuses on the general and the deliberately vague; and this works fantastically given the style of the game. There is no budget with which to be concerned, there are no apparent city divisions or districts. The placement of stations is random, and also out of the control of the player. In many ways, this almost makes the goal more true to life of a real world occupation, albeit without feeling like work: somebody is presenting the player with a scenario, and the goal is to make it work by connecting the stations, and keeping people moving.
The player begins the game by selecting one of three difficulties, and then the city that the player would like to play. These cities are mostly without features, save for waterways. The player looks upon either an off white void or sort-of-charcoal, if night-mode is turned on, with three subway stations to connect. The goal is to connect these stations in as efficient a manner as possible by drawing lines to connect them using the limited number of lines, cars and carriages that have been provided. With the initial three stations this is a simple task. More stations are added as the game progresses, which serves to complicate matters. Where the player may have easily begun using only a single line to connect all of the stations, this method will lead to disaster if it is maintained. The player will have to add new lines, new cars, add on carriages to those cars to ensure that no station overfills; and all of these are in limited supply. At the end of each week the player is granted an additional car, and then the choice between one other addition: tunnels, which are required to traverse through waterways, carriages, which extend the length of a train, lines, which adds a coloured line with which to connect stations, and interchanges, which increase capacity and speedily load and unload passengers. Only two of these additions are presented as options. The stations are randomly placed with each play through, as are the choices of additions, so memorization will not help the player. A set of options are presented, and the player has to make that work.
The stations themselves are without names, and are instead represented by shapes. Most commonly, these shapes are squares, triangles and circles, and occasionally 'other' shapes will appear that one could regard as special stations. The idea is that a "person" who boards at a circle station will never wish to travel to another circle shaped station. These individuals are also represented by shapes: a circle shaped passenger wishes to travel to a circle shaped station, a triangle passenger wishes to travel to a triangle station, and so forth. Therein lies the strategy: a line consisting of nothing but circle stations will lead to disaster as no passenger will wish to travel from one station to another. The player must try to connect as many differently shaped stations as possible. There WILL be some overlap and stations of the same shape connecting to one another; but the goal is to minimize those occurrences to maintain efficient routes and prevent overflow.
The game begins in a very calm fashion. Simply draw lines, add lines, extend lines, watch the trains pick up and deliver their passengers. It is that way for a while: calm. And then it isn't. All players will reach a point where chaos starts to creep in. Maybe entire lines, previously perfectly placed, are no longer efficient and have to be removed and rebuilt. As the player cannot control where new stations are placed, and what form those stations will be, new stations may spawn close to lines that already have a pair of stations that are the same shape on either side of the new addition; and that reduces the efficiency of the line. The alternative is to extend one of the other lines out to this new addition; but the player cannot cross the lines except by meeting them up at existing stations, which will make that extension even longer in the event that it does need to cross a line to access the new station. While such choices represent a challenge, that challenge never feels to me to be particularly stressful. The presentation of the game simply does not allow it.
The end result could be orderly, almost a work of art where transit maps are concerned; or it could look like a scattered mess of random connections; but so long as the stations never reach capacity then presentation hardly matters to the game. It may matter to the player, mind you. During one play through I had a well functioning three-lined transit system that crossed at odd intervals and zigzagged to and fro. It worked, and I hated it. It just didn't look proper. I had nobody to blame but myself: I COULD find another way for the system to work, but I had not looked for one.
The visual presentation is minimalistic. This is a good thing. Its appearance is that of a subway/metro map. The blank background populated only by single, thick lines that represent the metro rails that the player is placing and moving about serve well to illustrate the real transit maps that are out there in the world. The player is, in essence, painting the metro maps that one would see at subway stations. It is perfect, and immediately familiar to anybody who has found themselves staring at a map, plotting their subway route and transfers to progress from point A to point B. That said, those who have never ridden a subway, or are simply unfamiliar with subway/metro transit maps may be at a loss as to why this game's visuals are as they are, and it may look to such individuals as little more than lines on an empty space without knowing or appreciating the context as to why that look may be fitting. This is a fault of nobody, only the result of individual experiences.
Audio. There is none. There WILL be some; but at this stage of development it has not yet been implemented. If that's a turn off for you, then back away. Otherwise just fire up some music or background noise that you feel is appropriate, and enjoy the completed music and sound effects once they are added in.
Lack of audio aside, everything, for me, just felt right. The challenge is perfect, the visual style is appealing and fitting to the context of the gameplay, and the package as a whole has a polish to it, both because of and in spite of its minimalism. It is a game as casual as the player wants it to be with its difficulty settings, and as challenging as they can handle until the end finally comes as one station finally reaches over-capacity. I recommend it to anybody who is looking for something with few rules to remember, that is satisfying, and well put together.
Edit: one of the most recent patches (as of this writing) added interchanges as available possible options for upgrades at the end of each week. I added a description of these to the paragraph that detailed these add ons.