Train Public And Cargo Transport
Train Station and Cargo Train Terminal are your friends when it comes to growing your city and having a lot of commercial or industrial zoning.
Train Station will handle passenger traffic only. People moving into your city and tourists visiting the city can come through passenger train station. Usually when you build high density residential zones in large areas, many trains full of people will arrive to the nearest passenger train station.
Cargo Train Terminal handle raw materials and goods import / export only. This is the work horse for the industrial and commercial zones. It is a very good idea to place cargo train terminal next to your industrial zone so raw materials can be brough right there and excess goods exported. Any goods going to commercial zones will be delivered with box trucks and vans, usually its not suitable to have a cargo train terminal next to a commercial zone unless its some special mega shopping district setup.
Passenger and cargo trains can also have track network only within the city, no outside connection. If you have generic industrial area and commercial area(s) far apart, you could use cargo train terminal to haul cargo there. If your city is huge, you can connect different areas / neighborhoods using passenger train tracks if distances are so long that metro is not suitable.
Make sure train track outside connections are linked to the "terrain railroad tracks" with enough space between the "links" so that a big train traffic jam will not be caused by these links. Two train track connections (difficult to explain) should be far enough apart that at least one train can fit into the tracks between (sorry, really difficult to explain without images).
Always create passenger train station when increasing population, don't make these people drive private cars through highway.
Commuter train carries 400 passengers. High speed train carries 300. Passenger train (yes named like that) carries 240.