Good ideas.
Also for me I would like to see:
In the “Line” window as well as “Overview” and “Finances” tabs it would be good to have “Vehicles” tab and this should have a list of all vehicles running on the line, so you can double click one and open/view it.
In the “Vehicle” window, the assigned line should be shown on the overview tab. And in the Assign/set “line” tab then you should have an option to filter by ‘visible’ lines only as well as only showing lines for the specific type of vehicle (instead of greying them out).