I have found that the best system is a simple out and back.
If you have say four cites (in roughly a straight line) A, B, C & D, build a direct double track line to serve each of them as directly and by the shortest route possible. Then map the line as A>B>C>D>C>B (the trains will go back to A from B, so no need to map it). Build your lines to serve as many cities as possible that lie in roughly in alignment, they will attract the maximum number of passengers.
Circular line don’t work that well in either real life, or Train Fever!