A woman leapt from a moving train traveling from The Bronx into Manhattan and miraculously survived — thanks to a quick-thinking Good Samaritan who pulled the emergency brake, authorities said.

The victim jumped from a 6-train between 125th Street and 128th Street around 12:45 p.m., Monday and had her arm amputated by the train, officials said. She also suffered severe cuts to her leg, torso and face.

The 37-year-old woman was rushed to Harlem Hospital, in critical-but-stable condition.

Police said she had a fight with her boyfriend before she jumped.