The Train Simulator Package Manager feature is typically how a user can add or remove 3rd party addons (such as extra routes, scenarios, trains-sets or various other assets).
From the initial Train Simulator Utilities window, click on the "Package Manager" tab.
To install a RailWorks addon package, click on the "Install" button, then navigate to the addon file wanting to be installed (usually this is file will have a ".rwp" or ".rpk" extension).
After selecting the file to install you may see an installation progress-bar appear. Once the installation is completed, that addon should now be available to use once RailWorks is started. You may also notice in the far-left pane in the list of addon packages now should also display the name of the newly installed addon package.
To un-install an addon, from the far-left pane click to highlight an addon package then on the far-right side click on the "Uninstall" button. After a moment this will remove that selected addon from that RailWorks installation.
After installing or uninstalling an addon package, is it good practice to click on the "Tools & Docs" tab from the RailWorks launcher window, then to click on the "Clear Cache" button.
Optional ways a user may also be able to remove / delete a particular route or scenario addon can be found from the RailWorks in-game main user-interface menu, but this method does not allow for removal / deletion of addons such as Train-Sets or other extra various assets.