It is also possible. As written in the guide, the 20minutes traveling time is a maximum the people will tolerate. If there is no way for them to get to their destination in 20minutes they will not even go!!! So for some long train routes at the beggining of the game (slow trains) this may be a big problem. If it takes 5 minutes for a train to go from one station to the next it means it it arrive each 10 minutes to the station. Making the total travel time 15minutes. So it will be interesting only to people who will manege to get to the departure station + from the final station to their destination in 5minutes total. So you can serve only the close area.
If you add another train, you will reduce waiting times to 5 minutes. Total travel time will be just 10minutes. So you can serve much larger areas. Because people will use tris line even if they spent more time reaching it.
Also adding bus station near the train station that can bring peole as fast as possible fro. Other side of the city can realyy help. If they can reaxh your train station by walk in12 minutes they will not use it. If you provide bus that bring them there in 5 minutes, they will use it.
Very simple and kind of realistic.
this is what I really like about this game. It is not about placing stations just to cover the area. But about planning usefull lines that will primary help people reach their destination in shortest time possible.