Metro connects residential areas to commercial, industrial and office areas.
Pedestrian walking paths and buses are for short distances, metro is for medium to long distances, train is for a very long distances.
You can do two routes in one "track", clockwise and counter clockwise. You can also assign colors to the tracks for public transport menu so they are easy to see. Like blue for clockwise and red for counter clockwise. Green color is always the game default transport path.
In transport menu click the zoom glass icon to bring up the public transport lines menu. Click the color and then just pick the one you want.
Noise Pollution. Metro does give fair share of noise pollution, in real life one metro/subway train holds like 180 people, imagine how loud that kind of group of people is. Good idea to handle noise pollution is to put metro station away from residential buildings and have bus route carry people where they need to go, or you could surround metro with schools, office buildings and parks / plazas, even commercial buildings would do better than residential.
Metro generates noise pollution, the more traffic the more noise. Do not place metro station next to a residential buildings, surround it with office buildings or parks / plazas. Commercial generates noise too, so that is not the perfect buffer, its unconfirmed speculation but metro + commercial together might even amplify the noise pollution.
Its good idea to create metro track / route while game is paused. Game assigns number of vehicles for public transport route, if the route is very long more vehicles get assigned to it, you might want to start small to see if the route works properly and if vehicles fill up quickly then assign more vehicles to it.
Real life metro lines are not always loops, google "singapore metro map" which actually shows no loops, at all. They are just one line going from A to B and back again with many stations along the line.
When you place metro station near other sources of foot traffic, for example harbor or train station, place the station at the same side of the road as the source is, so the pedestrians walking don't have to walk a long way to cross the street. This is extremely simple thing to miss out on.
This header is about unconfirmed info from across the internet, these are not validated and who knows maybe they might be the greatest advice ever or simply not even work. Please keep that in mind while reading this header.
There is no point using metros in low density residential zones, instead use buses to gather people from this area to some centralized location which you place metro station.
You should not create more than 8 - 10 stations per one track network.
Avoid crossing more than two metro lines in a metro station, it most likely eventually cause traffic jams.
Avoid more than two lines in a metro track.
Do not duplicate bus lines with metro and the other way around.