After completing the Road to Freedom quest that leads you to The Railroad, speak to Deacon, the supportive guy wearing the sunglasses. He will ask you to meet him at the Old Highway in the northwest, as indicated by the waypoint.
Follow Deacon up the elevated highway to meet up with Ricky Dalton. Pay attention to Deacon's directions, especially the password needed to introduce yourself to Ricky. Don't worry if you mess up; Deacon can still vouch for you.
The waypoint indicator will lead you to a sewer tunnel, one of the entrances of the Switchboard, the old headquarters of The Railroad.
Travel through the many passages and rooms of the Switchboard, while dealing with Synths and turrets along the way. If you come across any doors or terminals you can't access yourself, Deacon will be happy to step in. When you reach door that resembles a giant safe, you have reached your destination. Let Deacon open the door, allowing you to retrieve the target item, Carrington's Prototype.
Return to The Railroad HQ and join Deacon during his debriefing with Desdemona. He's going to bend the truth to his boss; go along with his lie. Speak to Desdemona during an additional conversation to receive a codename, provided you've decided to join The Railroad. This will complete the quest and trigger the next assignment, Boston After Dark.