Steam Workshop Mods
In a recent update, Steam Workshop support has been added for Engines, Wagons and Custom Maps. Other mod categories are going to be added in a future update. You can check the mods you are subscribed to in the main menu inside the game. (screenshot TBA)
If you try to load a save file which uses Steam Workshop mods, the game will notify you when you try to load that save file, and will display a list of mods required to load said save file. (screenshot TBA)
Uploading a mod to Steam Workshop
Modding content categories
All other non-Steam-Workshop mods need to be manually copied into the game's directory to function.