Abstract: A fiber optic line magnetometer is used in a railway block detection system for detecting the presence of a train, or a portion of a train, within a predetermined block. The fiber optic line magnetometer is positioned in the center of the tracks and runs parallel to the tracks for the entire length of the block. The fiber optic line magnetometer includes a sense arm and a reference arm, each of which includes a fiber optic cable for carrying a light originating from a signal source. Magnetostrictive elements connect to the fiber optic cable in the sense arm and change the light path length when external magnetic fields are applied thereto. The light exiting from the sense arm and the reference arm are compared, and in the event the fringe shifts in the compared light exceeds predetermined thresholds, a detection control signal is generated.