I ADORE this game. It's arguably one of my favourite games of all time. Imagine Cities Skylines, but a beautiful, European city/town (based on their real life counterparts) is already completed and it's your job to build a public transport network. It was so ahead of its time, you can track individual cims through their commute and the attention to detail is incredible (every vehicle, animal, building etc. is simulated and can be examined, shows route if possible, background info etc.). You can build from the past (1900s) all the way to present and future times, and the vehicles and city structure reflect this. All the standard modes are here (I have all DLC, so not sure how many are in base game) - buses, trams, metro, ferry, helicopter/blimp, monorail (some maps), trolleycar (some maps).

You are given little missions to do along the way for rewards if you can like some structure, or you can go all out with total freedom and build a network with the widest coverage you can. Unlike Cities Skylines, stops actually show a radius for their coverage zone when building, you need to keep customers happy by running a good service (why don't they care at all in Skylines?), you can easily adjust the type/number of vehicles on a line and you need to take into consideration the pros/cons of each vehicle (fuel consumption, reliability, capacity etc.). It's not ridiculously easy like Skylines and you do need to build good routes (hey, just like real life!).

Get the DLC if you can afford (I'd argue it's necessary), be wary this game is resource intensive and install a few mods (manually, no Steam Workshop here) - must haves are ones that increase vehicle capacity to realistic amounts, or add a first person camera/more camera options to follow your little buses/trains around the city - see the guides for this game and also the Cim Exchange Steam Group. And avoid CiM2! Enjoy :)