BROTHERHOOD OF STEEL QUEST 12 - Tactical Thinking
Triggered by completing Brotherhood of Steel quest 11 - Blind Betrayal
Note: DO NOT undertake this quest if you want to side with The Railroad. Attacking members of The Railroad will make the faction (obviously) hostile to you, failing any current quests you are undertaking for them.
Note: The railroad leadership have powerful weaponry (a minigun, laser rifles etc) so make sure you are wearing good armour/ power armour on higher difficulties to survive.
Objective: Speak to Lancer-Captain Kells
When you arrive at the command deck of the Prydwen and talk to Kells, he'll inform you that the Brotherhood plans to take steps to wipe out The Railroad once and for all with you leading the charge. The Brotherhood aren't taking any risks with any faction that could jeopardize their operations against the Institute. Proctor Ingram will also add in her two cents, asking you to retrieve P.A.M the analysis assaultron from the Railroad's headquarters during your attack.
Note: Don't have Deacon as your companion during this quest. Place him in a settlement (such as Sanctuary) and remove all his gear. This will allow you to kill him easily as part of a later objective. Leave him tooled up and in power armour and he'll make mincemeat of you on higher difficulties.
Objective: Enter Old North Church
At this point you can teleport to the Railroad's front door essentially as you have already found them at least once before. Enter the church and take cover when you regain control because Doctor Carrington and a group of railroad members are waiting in the large main hall of the church.
Objective: Kill Doctor Carrington
The good news is that you will have Brotherhood support during your fight in the church and through the catacombs, so use that to your advantage. Use the AI allies as distractions and meat shields while you pick off the railroad members from range, or on lower difficulties just run in guns blazing. Carrington is only wearing a lab coat so one or two decent shots will finish him off. They didn't exactly send the best to the front lines here, meaning they are all waiting inside the main headquarters area.
Objective: Breach the door to the lower levels
To reach them, you'll still have to work your way through the railroad enemies scattered throughout the catacombs below the church first. Keep using your Brotherhood allies to make life easier as you continue your railroad attack. Take that subpar public transport system. When you arrive at the riddle disc where you needed to put in the password 'railroad' on your first visit, interact with it to plant explosives and blow the door down.
Objective: Kill Glory
Objective: Kill Tinker Tom
Objective: Kill Desdemona
Objective: Kill Deacon (he will show up at headquarters if you didn't bring him with you as a companion)
Keep fighting through until you arrive at the headquarters. Your first attacker inside the headquarters will likely be the drummer boy, so quickly make him bite the bullet. Then Glory and Desdemona will open fire. Glory should be your first concern thanks to the mini-gun she wields, which can quickly become a serious threat. Keep in mind that others can pick up the mini-gun after she dies as well, so make sure to keep an eye on it during the fight.
Desdemona should be next with her powerful weapon. Try to push forward here to escape the initial kill corridor you start in as you don't have any cover here. Try and reach the desks just inside the main room to stand a better chance of success. If Deacon is here, make sure he doesn't grab any of the weapons of other fallen enemies as he can be a pain to take down if that happens. The rest of the enemies here (as well as Tinker Tom) are nothing special and should go down easily due to their low damage resistance.
Objective: Reprogram P.A.M
Luckily for us, P.A.M doesn't actually join in the fight. So you can simply walk into her room in the back and use the terminal to insert the P.A.M decryption holotape and reprogram her after the shooting stops. You can destroy her if you wish, but there is no real benefit to doing so.
Objective: Report to Lancer-Captain Kells
Leave the way you came or via the escape tunnel at the back of the headquarters and fast travel back to the Prydwen to report the success of your operation and the end of this quest.