How to unlock: Overhear the conversation in Diamond City Market (near Nick's Valentine's quarters) (M6,1) or go to the place where the path starts.
The quest will start a chapter focusing on the Railroad faction, which are the opposite side to the Institute. It can be unlocked in a few ways. The easiest one is to overhear the conversation in Diamond City between two men located North from the main market square (the next block, near the "detective" sign that leads to Luck Valentine.
You can also go directly to the park in the city center that is marked with mission objective marker. There you will find a marking on the ground and a red line leading from it. It is quite long and not always visible, so you must be cautious. You can ignore the enemies you meet: ghouls, bandits and mutants. Just follow the path.
The path will end in front of the white door that leads inside the church. Turn immediately right when inside in order to avoid fighting ghouls. Walk through the door leading to the basement.
There you will notice a symbol on the last wall. It's a lock which can be turned in order to change letters. You must create a "RAILROAD" word from the letters on the face, confirming each letter by pressing the middle button. Once you do it, a secret passage will open.
At the start of the conversation (if you don't have the Charisma) you should pick up the lower option twice. This will allow you to easily complete the dialogue test.
Quest rewards: experience, Tradecraft quest.