Rail is a homunculus created by Huey Laforet as a test subject for his group "Rhythm". As a result of the experiments they have large stitch-marks running across their body and they lost their ability to feel the sensation of pain. Rail gets along with Nice because both of them have a love for bombs and they are presently traveling with Jacuzzi. Rail harbors a deep hatred for Huey. They are close to their fellow Laima, especially Christopher and Frank. They also have many interactions with Sham. Their relationship with Ricardo Russo appear to be strained, partially due to both of their relationships with Christopher, but Rail allows Ricardo to stay with Jacuzzi's gang in 1935.
Much like Ricardo, Rail is biologically female and refers to themself using the more masculine version of "I". Rail has told Czeslaw Meyer that they are indifferent to whatever gender they are perceived as, and thus are referred to in the article by singular "they" pronouns. Rail is stated to be 15 but appears 12 and does not age.