You want to convince commuters to take your train. Therefore you need to get them to the station as fast as possible, without unnecessary detours. They only have 20 minutes to get there where they have to go, so every minute counts! I use straight tram lines from city centre to the railway station.
Circular lines work well to get some money from people who commute inside the city, but on its own it is counter-productive for your train business.
Having both (straight lines to the station, AND circular lines for inner-city traffic) is possible, but without straight lines to the train station, your trains will suffer.
Just my experience and playing style (learned after playing only on hard mode). Everybody is free to play as he/she likes. There is no “enforcing law” to play a certain way. Just have fun!