Integra-Signum is a Swiss train protection system introduced in 1933. Originally it was called Signum; the name Integra was added later. It transmits data inductively and is simple, robust and reliable also in snow.
The locomotives have three sending and receiving magnets and there are two trackside magnets near the signals.
Integra-Signum asks the train driver to confirm distant signals that show stop and distant or home signals that show caution. If he does not confirm or passes a home signal that shows stop, the train is stopped automatically. This is achieved by interrupting the power supply to the motors and applying the emergency brake.
The locomotive's sending magnet is a strong permanent magnet, which induces a current in the receiving magnet in the middle of the track, if the signal's short-circuit contact is closed. The receiving magnet on the locomotive consists of two magnet field detectors, which detect the signal's state according to polarity and timing of the magnetic field emitted by the second magnet outside the track:
Stop (home signals): positive - negative, concurrent
Caution (distant signals): negative - positive, concurrent
Caution (home signal): positive - positive, not concurrent
Because Integra-Signum can only stop a train when it's "too late", i.e. after the red signal, it is not sufficient if there is an obstacle less than the braking distance away from the signal, which is especially a problem with fast trains. To address this issue, Zugbeeinflussung ZUB has been introduced. Both systems are supposed to be replaced some time in the future by the European Train Control System.
Despite that Integra-Signum aims to prevent accidents, there was also an accident caused by it. In 1959, an Integra-Signum magnet mounted on a SBB-CFF-FFS RBe 540 EMU tore out a wooden sleeper on a level crossing near Gland, which led to the derailment of the entire train at 125 km/h.