It would be absolutely amazing if the Steam version received Workshop support to allow fans to make their Journey through Sodor even more unique. There's no guarantee that every facet of the Thomas franchise will be featured in the game as future DLC like the Arlesdale and Culdee Fell railways (hopefully I'm wrong and they will be introduced), or such deep lore characters like Algy the Bus, Jinty & Pug, Ivo Hugh, Neil, BoCo, or Splatter & Dodge. To allow the game to have much further longevity for years, maybe even decades, Steam Workshop support would help to foster an ever-growing community of both train sim fans and fans of the Thomas franchise like we've seen with existing Thomas fan content for other competitive train simulation games over the years.
Pretty sure it will be a no as per Matt on why there is no Steam Workshop for TSW. With this being on so many platforms the answer will be the same
If no then hopefully modding will be easy to do just in case not every character and railway gets added to the game.