Train Simulator mods are being sold in quite a lively fashion, some people make decent supplemental income doing that. Naturally, those are pro and semipro-made models, textures and environments. Mods-for-sale is not without precedent, and Train Simulator has one of the most extensive and long-lived modding communities ever.
Now it doesn't hurt of course that the publisher of TS encourages or at least does not mind this, because it stimulates (in a roundabout way) purchases of TS DLCs (the entire library costs over $1000, and that's during Steam discount sales). Of course, paid mods get pirated and redistributed mercilessly, them's the breaks.
Most companies put the kibosh on selling derivative content though. I don't know what Larian's stance would be.