This interface should be implemented in order to receive railway crossing warnings. Note: The railway crossing warner can be either a zone warner or a point warner, depending on whether the railroad crossing warning is given for a railroad crossing zone or just a point. This means that for a railway crossing there will can be either 2 or 3 warnings emitted. In case the railroad crossing is a zone warner then 3 warnings will be emitted with the RailwayCrossingWarning.distance_type set to DistanceType.AHEAD, DistanceType.REACHED and lastly DistanceType.PASSED when the end of the railway crossing is passed. In case the railroad crossing is a point warner then 2 warnings will be emitted with the RailwayCrossingWarning.distance_type set to DistanceType.AHEAD and DistanceType.PASSED when the end of the railway crossing is passed.