To begin this quest, head to Goodneighbor at the east side of Greater Boston. Once in town, follow the path on the left and look for a narrow alley on your right (if you reach The Memory Den, you've gone too far). When you reach the correct door, a Ghoulish woman by the name of Bobbi No-Nose will offer you work. Accept the job to start the quest.
Talk to Bobbi about helping two other hires with an underground dig job. Meet the guys further down the tunnel. This job is not without its hazards, and the first you'll encounter are a few Mirelurks. It shouldn't be a big deal for you, but not for the two other diggers. It looks like Bobbi needs more help, and she knows the right man for the job.
Rendezvous with Bobbi at the center of Diamond City Market. She'll ask you to free a guy named Mel from the local jail. While you can steal the jail key or use a Protectron unit to distract the guards, the quickest way to free Mel is to bribe the guard, provided you have 300 caps to spare.
Converse with Mel after you've freed him and return to the dig site under Goodneighbor. Mel's not part of the crew for his digging muscle, but his robot digging partner, Sonya, is. She can use a sonic blast to obliterate loose walls, and there are several such walls between your current location and the other end of the tunnel.
While the tunnel is mostly a linear path, there are a couple of dead ends. Just be sure to investigate every wall for the sections that look like packed-in dirt. Sonya can clear out these spots. One particular dead end has power armor parts and a fusion core, should those items be of interest you. Your underground journey will also involve a couple of subway tunnels and some Ghouls.
You'll know you've reached the end when you encounter a Raider named Fahrenheit. You now have to decide whether to maintain your loyalty with Bobbi or side with Fahrenheit. If you betray Bobbi, kill her, then speak to Fahrenheit to complete the quest. If you side with Bobbi, kill Fahrenheit and the other Raiders. Then grab the train car key from Fahrenheit's corpse and open the train car. Speak to Bobbi to complete the quest.