When the mini air compressor is off, residual air in the compressor and associated
components can be unloaded by operation of a magnet valve in an unloader line so that the air compressor, when ordered to restart, does not have to start against such residual pressure. This protects the electric motor employed to drive the compressor, if the compressor is driven by such a motor, i.e., some compressors are driven directly off the diesel engine of the locomotive, as is well known in the railway art.