1. Talk to Desdemona
2. Talk to Preston
This quest becomes available only if your relationship with the Institute faction turns hostile at any point before Railroad Quest: Underground Undercover concludes. With no access to the Institute, there’s nothing more the Railroad can do to thwart the Institute. Desdemona says your best chance to stop the Institute lies with the Minutemen. Find Preston Garvey, and begin working with his faction instead.