I haven't been in the forums much today, been Trainzing myself.
Removing a DLC route has to be done carefully. You need to ensure you're not using the dependencies from that route in your routes, otherwise, you'll end up on a KUID hunt and have to replace those assets.
To prevent the redownload after removing the routes, you need to be careful when downloading assets unless you have Platinum and above. With the new Content Store, you can add and remove payware/DLC routes easily and not worry about downloading them again in error.
To remove a DLC route from TRS2019, non-Platinum and up, you need to find the package number. Go to the Packages folder where your content is installed - i.e. C:\users\(your name)\App Data\Local\N3V Games\trs19\some-build-number.
some-build-number = build with a bunch of numbers.
The package number is usually accompanied by a suffix - a letter such as SCC371, SCC371a, b, c, etc. (This is a made up number and not necessarily any particular one).
Once the folders have been removed, you need to run a database repair in order to remover any reference to that folder in-game, otherwise, things get weird.
Hope that helped.