I really liked the idea of train stations and building a transportation network with them. One thing I would change is to give them more adjacency bonuses with several other districts to make it more appealing to place them in city centers, maybe even give them a requirement to have at least x city tiles adjacent when placing.
I often found myself placing them at the very edge of the city when in reality major train stations are usually in the city center and cities fund very expensive projects for their construction and in some cases even tear down old buildings to place them in a more central place (or to expand an existing station).

Narcisse wrote:

Actually, it's rather rare (in Europe atleast) to install the stations network inside at the center of the cities. Most of the time, stations were builded on the different polarities of the city (north, south, east, etc...) and it was taken internally by the urban sprawl later with the natural expansion of cities.

Actually seems to be quite different from my experience. Several stations around here were built inside the city from the beginning. Often in places which had Infrastructure that was no longer required (commonly old defensive structures / city walls). After that they usually had to be expanded and even more spaced had to be repurposed.