Precipice of War

Precipice of War is a Railroad main quest in Fallout 4. It is the follow-up from Underground Undercover and is part of a chain of quests, all of which are tied together, to an alternative method of completing End of the Line.

Detailed walkthrough
''Note: This mission will have you do battle with a lot of enemies using energy weapons, and almost a dozen Brotherhood Soldiers in Power Armor. Make sure you are geared appropriately. ''

Railroad HQ

The Brotherhood of Steel sends a unit to assault the Railroad HQ. Their attack comes from the escape tunnel. Join the Railroad in the battle, and kill the Brotherhood unit. After they are dead, speak with Desdemona for an update on the situation. She will inform you that the catacombs are the only way out. She instructs you to join Glory, who already went ahead, in the battle.

As you exit HQ and enter the catacombs, you will find a wounded Glory at her dying breath. (Deacon approves if you tell Glory she'll make it) Regardless, she falls victim to her wounds, so it is left for you to secure safe passage.

Catacombs / Old North Church

You must now continue your journey through the catacombs towards the main floor of Old North Church. As you make your way through, a secret wall will burst open, revealing a Brotherhood Knight in power armor, and another knight in standard BOS Combat Armor. Kill them, and then continue your way through the crypt. More Initiates and Knights will be waiting for you, and some will be wearing power armor. Kill any BOS along your way towards the main floor of the church.

When you reach the main floor, another unit of Brotherhood Initiates, Knights, and Aspirants will be waiting for you. They will all need to be eliminated.

''NOTE: Depending on how you completed The Lost Patrol, Paladin Brandis could be among them. He will be equipped with either a minigun or a Gatling laser (randomly generated), and has a higher amount of hit points compared to the other soldiers. He is considered a dangerous enemy. After you kill him, you have a chance to grab the ultra-rare jet black version of the BOS uniform should you choose to. Additionally, his power armor frame is uniquely named, so shooting out his power core first will be to your advantage.''

Once all the Brotherhood soldiers on the main floor are dead, Desdemona and the rest of the Railroad will make their way upstairs to join you. After grieving over the loss of Glory, she states that the Brotherhood underestimated the Railroad, and thus their next attack will be far worse. She explains that the next step is to eliminate the Brotherhood threat as a whole: by destroying their flying fortress, the Prydwen. She then tells Tom that operation "Red Glare" is a go. However, he explains that it requires a Brotherhood Vertibird. Desdemona informs you that there is one on the Cambridge Police Station, and sends both you and Tom there.

Cambridge Police Station

Warning: ''When meeting up with Tom, it's not recommended to fast travel directly to the station itself, as you will end up spawning with exposed cover right in front of the Brotherhood soldiers, all of which are hostile. College Square, which is right around the corner of the station, is a safer fast travel destination. Either way, you have to meet up with Tom before proceeding.''

Outside of the station, after Tom finishes with his paranoia episode about the Institute possibly watching your every move, he explains that the Vertibird is on the roof as promised. He also says that the station needs to be cleared out of enemies.

At least two enemies will be in Power Armor. The rest will be in their standard Brotherhood uniforms such as Brotherhood Fatigues or Field Scribe's Armor. You need to kill every one of them outside and inside the station. Be sure to check the garage as there may be 2 BOS soldiers inside. Note that the garage door may be closed, so you will have to enter it from inside the station. Once they are all dead, make your way to the roof.

Tom explains to you that the Prydwen remains airborne because of bags full of hydrogen. He gives you an explosive charge and requests you to use it to destroy those bags and then return to the Vertibird to watch the "crash landing" from a safe distance. Afterwards, he proceeds to prep the vertibird for take-off.

While Tom preps, an enemy BOS vertibird will arrive at the scene and attack. Either kill everyone inside, or shoot down the Vertibird. Once that is no longer a threat, talk with Deacon to complete the quest. The quest Rockets' Red Glare will then begin.

Bugs

 * Tinker Tom will be sitting on the ground after taking too much damage and the quest will not progress unless he goes to the Vertibird. It will say "Defend the Vertibird" but Tinker Tom is bugged and so the attacking Vertibird will not appear to advance the quest. This can be fixed by fast traveling to another location, and then back again.
 * Shooting the attacking Vertibird with a weapon that can cripple an enemy's legs thanks to its unique prefix will cause the Vertibird to become indestructible - its health can be depleted but it won't crash, thus preventing the mission from progressing to the next stage. Make sure to bring a powerful backup weapon if the primary gear features this prefix.
 * Another way to fix it is to walk around and/or back and fourth below the Vertibird, this may cause it to crash.
 * Even if you shoot down the Vertibird, the quest can become bugged and not progress past "Defend the Vertibird". Entering 'setstage 2c8cb 11000' with the enemy Vertibird targeted will progress the quest correctly.
 * The church may be impossible to secure even after killing all the brotherhood members inside.
 * The setstage command also does not seem to fix this.
 * At Cambridge Police Station, killing some members of the Brotherhood of Steel will count towards your Murder statistic in your Pip-boy, despite them clearly being hostile towards the player.