Should I elect to allow the Silver annual rental to expire will these Trainz Content Vault items be blocked?
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Yes, or more accurately "access will be removed"
Will they be erased?
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No, they will remain installed in your /packages folder in your local data folder and will appear as "Installed, Payware (Not Active)" in Content Manager.
Will I forever be prevented from erasing them due to payware status?
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You can manually delete DLC payware by deleting the folders from /packages and running a db repair. Or if you have Trainz Plus, you can uninstall from the Content Store window.
Leaving the items installed an not activated isn't too different to leaving a book on the bookshelf. It takes up a little bit of room, but as long as you've got room on the shelf (or in this case on your hard drive), there is no harm at all in leaving it there.