Does that mean stuff outside DTG and Steam like from RSSLO or Virtual Railroad or some other company?
Stuff from outside DTG - Steam does not make any DLC for the game. This on the First Thread on page 1 - https://forums.dovetailgames.com/threads/third-party-repository.3548/ There's a lot of small third party sites out there that provide Train Simulator with all sorts of content from routes and reskins to scenarios and scenery. Some of these third party groups are well known and provide content through payware such as Just Trains or Armstrong Powerhouse. Some are not as well known and provide freeware like trainsimdesigns.co.UK or DPS simulations. I think it would be of great benefit to everyone if we could compile a list of all these sites and what they offer.
There are five categories as I see it.. 1) Official DTG Sold (through Steam) content, this is the content you will find when you search for Train Simulator Classic on the Steam store, note I said "DTG Sold" content because official content isn't solely that which is produced by DTG many "3rd party" developers content is sold through DTG and their Steam store pages i.e. Just Trains, Bossman Games, RSSLO, Rivet and so on (this category also includes Steam DLC keys available from the likes of HumbleBundle and other Steam key selling sites). 2) Third Party Payware developers that do not sell their content through DTG and their Steam store, these developers have their own website to sell stuff that will be directly downloaded to you without Steam being part of the process for example Armstrong Powerhouse, Alan Thomson Simulation and too many others to list here (Just Trains falls into both categories here but be aware as their are subtle differences between the Steam sold stuff and their own website sold stuff, namely branding and other 3rd party payware included in their scenarios) 3) Third Party Freeware These are the hard working talented community members that create content (routes, models, scenarios) because they love doing it, it is their hobby their reward is not financial (some except donations though this is never compulsory) it is the enjoyment of creation and sharing and undoubtedly the warm feeling of knowing they made something that many others will enjoy too. Also many Payware developers also release freeware content too and vice versa 4) Steam Workshop This is very similar to the Freeware category apart from a couple of differences, a) Obviously this content is only available via the Steam Workshop page and download system which you simply subscribe or unsubscribe from to get the content and b) there may be certain rules/guidelines to follow when uploading to the workshop, however I'm completely the wrong person to elaborate on this as I know nothing about the Steam Workshop other than the few route merges I've subscribed too. 5) Community tweaks and fixes Yes this could be grouped into the 3rd Party Freeware category but I feel it's significantly different enough to warrant it's own category because these are usually just small file edits to either some scripting or textures and they are quite often shared directly via forums or Facebook though many websites like ATS, DPSimulation, VP and so on do have sections dedicated to these small tweaks and fixes.
If you are asking about this week's screenshot contests theme, this rule still applies... Basically only 3rd party developers content that is available through steam only.
