Think of it like SpacecChem. You need to manage the input (passengers) and output (getting those passengers to stations). As each stage progresses, you get more stations and need to use your limited amount of bridges and cars to shuttle the passengers around efficiently, while making sure stations don't get overcrowded.
You need to strategically consider your line placement. Do you make long tracks that pick up a lot of passengers but takes longer to reach stations? Shorter loops that reach less stations? Where do you place bridges and tunnels? Which upgrades will best suit your map and current track layout?
Also don't be afraid to erase some tracks as the stages get bigger and you have more cars, bridges, and stations to use. Your early set-up will not be effective on a larger scale
It took a while for the game to click with me. Don't think if it as trains and passengers, but more abstractly: building an efficient network for transporting resources and then optimizing under pressure and tight limitations,