Q: Will I lose a saved scenario if I play anything else (a new route/train, quick-drive) other than clicking 'Resume' every time I launch the game? Or will it stay available as long as I don't save my place in any other scenario I may play in between times?
A: Don't worry. Each scenario is saved in its own folder.
Just find it in the game (select "Main menu", "Single Player", "Standard scenarios"), select it, click on "Go", answer "Yes" to "Resume".
The "Resume" option in the Main menu is just a shortcut for "Resume the last saved scenario".