Is anyone else having an issue where TSW4 starts installing when you buy a DLC for TSW5? I've had start installing with my last two DLC purchases and have had to intervene manually.
That is normal when purchasing a "TSW x compatible" DLC. You are entitled to all game versions of the DLC, that's not due to Steam but due to DTG's DLC structure. Steam will automatically install purchased DLC to the game it applies, and in this case, it's valid for both games. If you uninstall TSW4, this won't happen of course, else just pause the download, got to TSW4 DLC Manager and untick it.
I don't have TSW4 installed though, yet both times it still tried to download it when I bought the DLC.
Hello there, when buying a product on Steam you are prompted to download your new purchase in a popup window. I suspect you are choosing "Yes", which means Steam will start the download, but since it is a DLC, it will also start to download the base game, which in this case is Train Sim World 4. If you have TSW 5 installed you won't need to manually agree to downloading your new purchase, as it will automatically add and download the new content to TSW 5. I hope this helped. All the best, Jan