Mashinky modding
The easiest way how to start modding is adding some new content (3d asset, industry chain, sound, etc.) and enlist it in game configuration files. You can also create scripts.
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. Due to Early Access, the Workshop page is only accessible if you are logged into a Steam account that owns the game. 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
(TBA)
Modding content categories
All other non-Steam-Workshop mods need to be manually copied into the game's directory to function.