First Time User steps:

1) Turn on mods in the Mod Settings panel, located in the lower right of the Main Menu.
2) Subscribe to Arcadian Clan
3) Subscribe to Trainworks (there's a link on the right sidebar - Steam may auto-subscribe you)
4) Boot up the game
5) Double check in the Mod Settings panel that Trainworks and Arcadian Clan exist and are turned on

Still having trouble? Join the discord in the description, or ping me on the official Monster Train Discord for the fastest support.