I begin with taking photographs of the map with PrScr and piece together the map ( have not figured out if the game can dump out the whole map in one go )
Then I mark all cities and factories. I made up my own symbols. Resources hollow symbols. Factories filled symbols.
City: Hollow 5 edged star with yellow fill so it shines more.
Forest: Hollow triangle -> Saw mill: Filled triangle
Coal mine: Hollow square + Ironmine: Hollow Square with a plus sign in it -> Steel mill: Filled square
Oil well: hollow circle -> oil refinery: filled circle.
There are more resources and factories on the map then cities. So you begin with drawing lines between resources and factories and cities. No production will start until there is an offset for the goods. You connect as many short distance lines first.
It is a puzzle. After you been drawing for awhile you get clusters of cargo routes, You then see more easily where there are gaps and your passenger lines can snake their way through the landscape without tunneling under or over cargo rail lines.
You do it this way there might only be 1 – 3 places on each map where the rail lines have to cross.
It is better to plan well and be able to solve the entire map then build 50% and then run into tons of problems when you try to add more.
I love freight trains. Hauling gigantic loads of cargo is my goal. That is where railroads make their money. And then some on passenger trains too.
( would be nice if you could set the ticket price on the lines to suck in passengers just so that other lines can be profitable. )