Bethesda had patched the Steam version of Fallout 4 to 1.7.15, the express purpose of which is to fix the Nuka World expansion's busted monorail system.
In fact, that's all the patch notes say: "Fixed issue with Nuka World monorail not starting properly."
Bethesda plans to release the patch on Xbox One and PS4 next week.
It's been a busy week for Fallout 4 fans. Nuka World, the sixth and final DLC expansion for Bethesda's 2015 post-apocalyptic RPG, was released; and Nvidia announced a mod titled Vault 1080 for release today—marking the GPU company's first foray into game development.