THE RAILROAD QUEST 05 - Precipice of War
Triggered by completing The Railroad quest 03 - Underground Undercover
Note: You will be dealing with Brotherhood enemies in power armour during this mission so bring weapons that are effective against them (their lowest resistance is to energy weapons) and/ or your own suit of power armour.
Note: If you have Danse as a companion, after this quest he will become hostile towards you.
Objective: Defend Railroad HQ
As soon as you finish telling Desdemona about the incoming Brotherhood attack, the whole place will start shaking as railroad agents run about preparing a defense. Around four or five Brotherhood knights and scribes will burst in through the tunnel entrance to the Old North Church and start engaging Railroad personnel. A good strategy is to stay on the opposite side of the room and fire from that position to avoid back and flank attacks. The other Railroad members also act as decoys while you engage in this firefight.
Objective: Talk with Desdemona
After the final Brotherhood enemy bites the dust, Desdemona will tell you that your only option is through the Catacombs leading back to the Old North Church. These are now (obviously) swarming with Brotherhood of Steel troops, so it's not going to be a walk in the park.
Objective: Secure the church
Enter the doorway leading to the catacombs and you'll find Glory huddled up against a wall in the same room where The Railroad originally met you for the first time.
Note: Apparently Glory can glitch out here depending on your faction progression, acting like a lifeless doll or even surviving. This doesn't have any impact on the actual quest though. Just kinda weird.
After talking to Glory you'll need to push through the linear catacombs through the Brotherhood forces. Take advantage of cover when you can, make sure you engage enemies one at a time whenever possible and use effective weapons against them (energy) if you can. If you are wearing power armour, this really shouldn't be difficult. Near the stairs leading up to the church is a power armour wearing knight with a mini-gun who can be a problem, but a couple of grenades to destroy his armour before attacking him can really soften him up.
The last of the invading Brotherhood forces will be within the church itself, so be careful as you ascend the stairs as around four enemies will be waiting above. A strong sneak attack or explosive weapon (missile launcher or mini-nuke) will cause serious devastation here. Polish off the remaining forces and Desdemona and Tinker Tom will arrive at the church shortly afterwards.
Objective: Talk with Desdemona
Desdemona will tell you of The Railroad's contingency plan against Brotherhood attack. Blow them up. Not complicated, but it could be effective. To do so you'll need to commit grand theft vertibird first though.
Objective: Meet Tinker Tom at the police station
Note: Teleporting straight to Cambridge Police Station will place you in the middle of a firefight. To avoid this, you'll want to teleport to a nearby location to collect Tinker Tom before attacking the station (hey, an extra gun is always handy).
Talk to Tinker Tom to get him to accompany you and then head for the police station.
Objective: Eliminate Brotherhood forces
You'll be facing serious Brotherhood opposition here (eight plus enemies with at least two in power armour), so make sure you're packing before starting the rumble. One lone red marker may remain in the area, this is likely a final enemy who is inside the parking garage of the police station. You can access it from inside the station if you haven't opened it already. More Brotherhood await you inside as well, so continue to mow them down. When the whole station interior and the area just outside is free of Brotherhood enemies, turn to Tinker Tom to update your objective.
Objective: Talk with Tinker Tom
Tinker Tom will fill you in on the remainder of the plan and tell you that he needs time to prep the vertibird (and work out the controls). He'll also hand over some explosives which you'll be using in The Railroad Quest 06 - Rockets' Red Glare. Unfortunately, an incoming vertibird is bringing in reinforcements as your talk concludes.
Objective: Defend the vertibird
There are two ways to do this (depending on your arsenal and accuracy). You can either wait until the incoming vertibird disgorges it's passengers onto the rooftop and then kill them, Or with a powerful weapon (missile or railway rifle for example) you can deal enough damage to the vertibird to destroy it, taking out it's passengers with it. Either way, when it is no longer a threat you'll have another quick chat.
Objective: Talk with Deacon
Deacon will hand over several goodies to prepare you for the upcoming battle against the main Brotherhood forces. These include a plasma rifle, x6 plasma cartridges, a pulse grenade, x2 plasma grenades, x2 stimpaks and a stealth boy. After receiving the haul, the mission will come to a close as you prepare to take on the Prydwen.