Main. Having said that, there are other points that also make you hostile with other factions, sometimes with no warning. Theres a Brotherhood quest, I forget the name, but its after the
stuff. When you have the quest and go talk to Kell, it imediatly puts all your Railroad quests in a fail state, without warning. Just having the quest is fine, talking or even going near Kell with it is not. Thankfully I save like every 5 minutes so I was fine.
Edit: Tactical Thinking is the name. Not even going to spoiler that because it spoils nothing and people should be aware