Just had an idea - I don't think you should be able to connect cities to each other by railroad unless they have a railway Station.
Secondly - there should be difference between main railway lines linking city to city - and just mass transit lines for the city use around the other city squares.
Eg- to connect City to city you will have to build railway Station.
To build railroads or local railways around the other city square - one must build the mass transit improvment.
Also - there could be a correspondance with how many railway stations you have, and how much trade is generatedd as well as other things that may be also included or effected by this.
Secondly - there should be difference between main railway lines linking city to city - and just mass transit lines for the city use around the other city squares.
Eg- to connect City to city you will have to build railway Station.
To build railroads or local railways around the other city square - one must build the mass transit improvment.
Also - there could be a correspondance with how many railway stations you have, and how much trade is generatedd as well as other things that may be also included or effected by this.