Tradecraft

Tradecraft is a Railroad main quest in Fallout 4.

Detailed walkthrough
The quest begins immediately after completing Road to Freedom unless the player character has already started the quest The Molecular Level. This gives the player character the option to go inside Railroad HQ without joining the Railroad and starting this quest. Going to the Railroad through The Molecular Level will change Deacon's dialogue and refer to the player character as the "Courser Killer". No matter what the player character says to Desdemona and how she responds to them, Deacon forces her to let the Sole Survivor inside HQ.

The conversation with Deacon will change depending on the answer to Desdemona's question in the previous quest regarding whether the Sole Survivor would risk their life for a synth.
 * If the Sole Survivor answered yes, Deacon will joke about "considering it a close personal favor" if the Sole Survivor did not sell them out to the Institute, after which he will provide details on the job.
 * If answered no, Deacon will ask if the player character would at least help take the fight to the Institute for their crimes against the Commonwealth. Saying yes to this will lead to Deacon providing details on the job.

Either way, Deacon has an angle for getting past the usual, year-long procedure of becoming a Railroad agent, suggesting a two-man mission to the Railroad's former hideout in Lexington. Meet him on the city's outskirts to continue the mission.

Outside Lexington, Deacon provides more details on the job, introduces the Sole Survivor to a few railsigns, and tells them the active Railroad countersign ("Mine is in the shop") to use with Ricky Dalton, a tourist they are about to meet. They rendezvous with Ricky, who greets them with the Railroad's sign "Do you have a geiger counter?", to which the player character can choose whether or not to use the countersign. Ricky will then explain that the Slocum's Joe which served as the Railroad's former headquarters is crawling with Gen-1 synths.

After receiving this information, the Sole Survivor has the opportunity to persuade Ricky to provide them with assistance in the upcoming mission via Charisma checks of varying difficulty. Assistance can take the form of extra supplies (Easy), sniper fire when the violence starts (Medium) or an explosive distraction (Hard). Ricky notes that the distraction will likely kill him, at which point the player character can still tell him not to do it and he will resort to providing sniper fire instead. Failing any persuasion attempt will cut off all others, and passing the medium or hard before the easy check will cut off that option as well. If persuaded to do create an explosive distraction, Ricky will walk to near the Slocum's Joe and sit on a large rock and do nothing more. He is highly likely to be attacked by mongrel dogs or Gunners on the way and die.

After speaking to Ricky, Deacon will speak to the Sole Survivor and outline the two options for retaking the Slocum's Joe - a frontal assault or a rear assault via the escape tunnel.

Option 1: Frontal assault
Proceed to the Slocum's Joe front entrance and begin a frontal assault. There is a large field of frag mines at the front of the shop which will first need to be circumvented or disarmed. Once inside, Deacon will give the Sole Survivor the location of the hidden elevator only to realize it has been powered down, forcing the player character to use Option 2, sneaking in through the escape tunnel. (Before update 1.4 it was still possible to enter this way by opening the elevator doors, jumping down the shaft, entering the elevator box via the hatch at its top, and opening the doors at the bottom.)

Option 2: Escape tunnel
The escape tunnel will be marked on the map, in a sewer tunnel covered by a thick drape of moss. Upon entering the tunnel, Deacon will give the Sole Survivor more information about the Railroad and the target. After the talk, Deacon will unlock the security door using a terminal and a few passwords he remembers.

Past the door, Deacon will point out a railsign signifying that there is a cache nearby. It is in the back of the tube directly behind the sign. Around here, Deacon will warn the Sole Survivor that they should just run if they ever meet a Courser (provided the player character has not previously fought one in the Hunter/Hunted quest). Ahead, there will be an unlocked terminal, which one can use to turn off the spotlights and turrets in this area. Below is a group of synths that need to be passed or killed. If the group is sneaked past, one can turn on the defenses that will start firing immediately against the next group of synths. Shortly after that, the Sole Survivor will discover that the Railroad was using an old Defense Intelligence Agency site as their HQ.

The terminal next to the door leading out of this office area can turn off the tripwires in the next room. Before leaving, grab the fusion core in the room left of where the Sole Survivor first entered and a U.S. Covert Operations Manual on a desk in the room up the stairs. A terminal in this room controls access to two locked areas containing minor loot.

Once entering the last room before the quest marker, Deacon will open the vault and inspect a dead body. There are two hazmat suits and a bottlecap mine in the room before the vault, opposite a weapons workbench. After the inspection, he will offer the "hand cannon" the dead man used and will not take no for an answer. Grab Carrington's prototype from the vault and head towards the elevator. This area has two Stealth Boys, a Mini Nuke and fusion core which can be looted.

If the Sole Survivor has not yet done so, there are synths and turrets topside which will need to be destroyed. If Ricky is helping, there is a good chance he will be killed providing sniper fire, although he can be saved, but guaranteed to die with the explosive mini-nuke distraction, which wipes out most of the synths. Deacon will suggest that the Sole Survivor and him split up and meet back at the Old North Church.

When the Sole Survivor reaches the church, they find Deacon embellishing their exploits to Desdemona. There is the option to tell Desdemona the truth or pass an easy persuasion check to go along with the lie. Either way, Desdemona is impressed with the Sole Survivor's resourcefulness and extends an invitation for them to join the Railroad. Refusing leaves the offer open, but accepting leads to selecting a code name (or letting Desdemona pick 'Wanderer' after refusing a name twice) and completion of the quest, with Deacon becoming available as a companion.

Companion reactions

 * Curie likes it if the player character declines the gun (the player character will still receive the gun). Cait dislikes declining the gun.
 * Curie likes it when the player character thanks Ricky for the information. This ends the conversation, though you can talk to him again for the cover-fire speech checks. You can elicit another like from Curie by convincing Ricky to provide explosive support, then telling him not to do it.
 * Deacon loves it when the Sole Survivor joins the Railroad.
 * Piper, Nick Valentine, and Curie like it when joining the Railroad.
 * Danse hates it when joining the Railroad.
 * Cait and MacCready like it if the player character goes along with Deacon's lie. Piper, Preston Garvey, Nick Valentine, Codsworth and Curie dislike this.
 * Nick Valentine and Curie like telling Desdemona the truth. MacCready dislikes this.
 * Piper and Hancock will dislike telling Ricky to make the distraction.

Bugs

 * Upon reaching the stop under the Old Highway near Lexington, Deacon might get bugged and repeat the same lines and will not allow for dialogue to progress the quest. To resolve this, simply reload a save or dying can allow him to initiate the proper response.
 * Deacon might stall in the hallway outside the room containing the locked vault and the inaccessible terminal. This can be prevented by following and clearing the dialogue tree at the start of the tunnel. To resolve the issue:
 * Save and reload the game, and optionally attack and down Deacon before doing so.
 * Use followed by  on Deacon and he may proceed to the terminal.
 * Exit the Switchboard via the elevator and exit to the Commonwealth, upon re-entering the Switchboard via the elevator the vault should now be accessible.
 * While facing the door use console command to unlock the door:
 * Using the command should make Deacon walk over and use the vault terminal, continuing the quest from there.
 * Deacon might stall upon first entering the Switchboard, failing to move upstairs to access the terminal and unlock the double-doors (an issue if the player character does not have the Hacker perk). Open the console and left-click Deacon. Close the console and go stand directly in front of the locked doors. Open the console and type . The door should automatically open once Deacon is within close proximity.
 * When approaching The Old North Church to meet Deacon after getting Carrington's prototype, Deacon may suddenly move to the middle of the map making it impossible to meet with him. Loading a save game from the 'Get Carrington's prototype' step may solve this. Returning to the Switchboard to find and speak to Deacon there may also solve this.
 * Using on Deacon may also fix this issue.
 * The prototype may not be removed from the inventory after handing it over to Carrington. Talking to him a second time or, on PC, using may fix this issue.
 * Deacon might get stuck in mid air, unable to speak or move, after the player character has talked to him under the old highway. A way to fix this would be to target him, open the console, and type . This will fix Deacon floating around.
 * If one did not talk to Deacon after entering the back entrance and only speaking to him after obtaining Carrington's prototype, the quest log will prompt the player character get the prototype even when it is already in their inventory. And the quest log will not update even after the completion.