The Railroad

The Railroad is an underground movement with a primary aim of freeing sentient Synths from their creators at the Institute.

Fallout 3
There is little available information about either the Railroad or the Institute, but the Lone Wanderer becomes involved in their conflict during the quest The Replicated Man in Rivet City, where a scientist named Dr. Zimmer requests aid in capturing a runaway android, and Railroad agent Victoria Watts pleads with the character to take pity on the android and fake its/his death.

The Railroad's methods for aiding in escape can be extreme, going so far as to find doctors to perform full facial reconstruction on escapees. They are known to operate in the Capital Wasteland and at least as far north as the Commonwealth.

Dr. Zimmer, Ms. Watts, and the android give very logical and convincing arguments for their point of view, and the player character must make an ethical choice based on the limited information they gain from each of them. There are multiple "good Karma" solutions.

Fallout 4
The Railroad is a joinable faction situated in the Commonwealth who assist runaway synths. Their main base is located in Old North Church in the city formerly known as Boston. They maintain a network of safehouses and clandestine contacts reminiscent of their namesake, the 19th century Underground Railroad which helped slaves escape and reach free areas. Their main mission is to assist synths who wish to escape The Institute who use them as manual labor and slaves.

The Sole Survivor can learn about the Railroad through a few media. You can investigate the area around Park Street Station, speak with Doctor Amari at the beginning of The Molecular Level who indicates that the Railroad would be helpful to your cause, or overhear people in the Diamond City Market, the Dugout Inn, or guards in Goodneighbor discussing the elusive Railroad. All of these will result in the Sole Survivor getting the quest Road to Freedom. Piper also makes comments about them after the The Glowing Sea quest starts.

The Railroad is joinable through the Fallout 4 quest, Road to Freedom.

Membership in the Railroad may negatively affect relations with other main factions. The more quests the Sole Survivor does for the Railroad, the more the Institute and the Brotherhood of Steel will disapprove of their actions. If the Sole Survivor sides with the Brotherhood of Steel or The Institute, they are sent to exterminate The Railroad, believing that they will try to stop them.

Relations with the Minutemen are not affected by joining the Railroad. However, when completing the Minutemen version of The Nuclear Option the optional mission must be completed or the Railroad (including Deacon if he is your companion) will become hostile when the player next speaks to Desdemona. You will still be able to accept A Clean Equation missions from P.A.M., Lost Soul mission from Doctor Amari, and even the To the Mattresses missions from Desdemona when Drummer Boy notifies you. Any extra attempt to speak with her outside accepting and completing quests will cause The Railroad to become hostile. This appears to be a bug as the Lost Soul missions should not be available if you left no survivors at The Institute.

Appearances
The Railroad appears in Fallout 3 and Fallout 4.

Behind the scenes

 * The Railroad is modeled after the "Underground Railroad", an organization in America in the nineteenth century who dedicated themselves to helping African-American slaves escape to the north.
 * The Railroad contains a number of similarities to Skyrim's Thieves Guild (an agent named Maven, the name 'Mercer' in Mercer Safehouse, brick-walled underground home bases, the use of symbolic markings on walls around The Commonwealth, etc.).

Railroad El Ferrocarril Железная дорога