code: def write_temporal_networks_by_route_type(gtfs, extract_output_dir):
      
      util.makedirs(extract_output_dir)
      for route_type in route_types.TRANSIT_ROUTE_TYPES:
          pandas_data_frame = temporal_network(gtfs, start_time_ut=None, end_time_ut=None, route_type=route_type)
          tag = route_types.ROUTE_TYPE_TO_LOWERCASE_TAG[route_type]
          out_file_name = os.path.join(extract_output_dir, tag + ".tnet")
          pandas_data_frame.to_csv(out_file_name, encoding='utf-8', index=False)
docs: Write temporal networks by route type to disk.
  
      Parameters
      ----------
      gtfs: gtfspy.GTFS
      extract_output_dir: str