diff --git a/src/engine_func.h b/src/engine_func.h
--- a/src/engine_func.h
+++ b/src/engine_func.h
@@ -26,6 +26,7 @@ bool IsEngineBuildable(EngineID engine, 
 bool IsEngineRefittable(EngineID engine);
 void GetArticulatedVehicleCargoesAndRefits(EngineID engine, CargoArray *cargoes, CargoTypes *refits, CargoID cargo_type, uint cargo_capacity);
 void SetYearEngineAgingStops();
+void CalcEngineReliability(Engine *e, bool new_month);
 void StartupOneEngine(Engine *e, Date aging_date);
 
 uint GetTotalCapacityOfArticulatedParts(EngineID engine);