Precipice of War Main Story, The Railroad Quest
This quest will be triggered or can be aquired when you complete Underground Undercover quest.
Related points of interest
Stages of "Precipice of War" quest
1. Defend Railroad HQ
I got to Desdemona in time. But the Brotherhood of Steel could attack at any moment. I have to defend my friends.
3. Secure the Church
There's only one way out of the HQ - straight through the catacombs and everything the Brotherhood brought to wipe us out. I'm the Railroad's only hope.
5. Meet Tinker Tom at the Police Station
Railroad HQ survived. But until we destroy the Brotherhood's Prydwen we'll never be safe again. To stop them we need to acquire a Brotherhood vertibird at Cambridge Police Station. Tinker Tom will meet me there.
Tinker Tom waits for you at the College Square.
6. Eliminate Brotherhood forces
We need to secure the Cambridge Police Station so Tinker Tom can commandeer their vertibird. That vertibird is the key to destroying the Brotherhood's flying fortress, the Prydwen.
Kill all the soldiers outside and inside the Cambridge Police Station building.
8. Defend the vertibird
Tinker Tom's going to need time to get our commandeered vertibird flight worthy. So Deacon and I have to keep him safe until then.
9. Talk with Deacon
With the Brotherhood forces dealt with, Tinker Tom has the vertibird ready for take off. Now I need to check in with Deacon before we board her.
10. Quest completed
With the Brotherhood vertibird secure, we're as ready as we'll ever be to deal a fatal blow to the Brotherhood of Steel. By destroying the Prydwen.
Next / new quest is Rockets' Red Glare (Main Story, The Railroad).