This should have minimal impact on Cities: Skylines, as the game isn't really affected by whether or not you are logged in to your PDX account. For us mods and assets are handled through Steam, so it's really just the Paradox Plaza park asset that might act up. And of course the forum will be unavailable for a while.
On the main menu of the game, does it show as owned on the list of DLCs on the left? If it does, then you are looking for this icon. The CCPs have their own little submenus, so all the assets are in one place. For Train Stations you will see this one under Public Transport, all the way to the right, next to the hubs.
https://i.imgur.com/jcDIMj9.jpg