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Acronym for "Induktive Signalsicherung", Inductive Signal Protection. This is a German railway device that prevents conductor error.

Passive resonators are placed on the tracks next to the signaling switches, linked to the stop/go signals. The locomotive have corresponding coils emitting magnetic impulses. When a resonator passes by a coil it can detect if the switch is on stop or go.

If the resonator is shorted i.e. does not affect the locomotive's active coil the signal is set to go. If the resonator is active, the coil will detect it and signal the operator to slow down, stop, or as a last resort activate the emergency braking system.