code: def combined_stop_to_stop_transit_network(gtfs, start_time_ut=None, end_time_ut=None):
      
      multi_di_graph = networkx.MultiDiGraph()
      for route_type in route_types.TRANSIT_ROUTE_TYPES:
          graph = stop_to_stop_network_for_route_type(gtfs, route_type,
                                                      start_time_ut=start_time_ut, end_time_ut=end_time_ut)
          for from_node, to_node, data in graph.edges(data=True):
              data['route_type'] = route_type
          multi_di_graph.add_edges_from(graph.edges(data=True))
          multi_di_graph.add_nodes_from(graph.nodes(data=True))
      return multi_di_graph
docs: Compute stop-to-stop networks for all travel modes and combine them into a single network.
      The modes of transport are encoded to a single network.
      The network consists of multiple links corresponding to each travel mode.
      Walk mode is not included.
  
      Parameters
      ----------
      gtfs: gtfspy.GTFS
  
      Returns
      -------
      net: networkx.MultiDiGraph
          keys should be one of route_types.TRANSIT_ROUTE_TYPES (i.e. GTFS route_types)