Maybe on some maps after a few weeks you could have station appearing (max 2 or3 maybe? I guess it depends on the city as London has 5 terminals: Paddington, Euston, Victoria, Waterloo and Kings Cross) that link to mainline train services. These would have a line running out of them that did not interact at all with the metro lines (no speed penalty if they cross) and they could bring in extra passengers and take certain passengers. They could have a special shape (hexagon?) and would have a larger capacity and transfer speed than regular stations but not as large as an interchange. They would not be able to receive an interchange upgrade. What do you think?