Road to Freedom Main Story, The Railroad Quest
This quest can start while listening to the residents of Diamond City Market talking about The Railroad. Just take a walk around the market.
This quest can also be triggered when you receive the main story quest - Unlikely Valentine.
Related points of interest
Stages of "Road to Freedom" quest
1. Follow the Freedom Trail
If I want to fight the Institute, I've been told I should find the Railroad. The only clue I have is: "Follow the Freedom Trail." The tourist trail starts at Boston Common.
Go to the Boston Common and locate Freedom Trial Marker, it's next to the fountain with a board that reads "At Journey's End Follow Freedom's Lantern".
2. Continue following the Freedom Trail
Follow the red bricks path, it will lead you to the final destination - Old North Church.
Each marker on the trail has a painted, red number (1-8), and one letter is marked with red.
3. Search for the Railroad
The Freedom Trail leads to the Old North Church. The clue said : "At journey's end follow freedom's lantern." Using the letters and numbers I found on the trail, hopefully I can find the Railroad.
Go down to the catacombs of the church and find Freedom Trail Ring. Use it (spin the ring to select proper letter and then press button) to enter RAILROAD. It will unlock hidden doors.
4. Talk with Desdemona
She will tell you that you should talk to Deacon and you will receive new quest - Tradecraft.
5. Quest completed
I did it. I found the Railroad.
Next / new quest is Tradecraft (Main Story, The Railroad).