fixed through track with new connection scheme

flatland/envs/rail_generators.py

@@ -776,6 +776,9 @@ def sparse_rail_generator(num_cities=5, min_node_dist=20, node_radius=2,
         """
         through_path_cells = [[] for i in range(len(node_positions))]
         for current_city in range(len(node_positions)):
+            all_outer_connection_points = [item for sublist in outer_connection_points[current_city] for item in
+                                           sublist]
+
             for boarder in range(4):
                 for source in connection_points[current_city][boarder]:
                     for other_boarder in range(4):
@@ -783,8 +786,8 @@ def sparse_rail_generator(num_cities=5, min_node_dist=20, node_radius=2,
                             for target in connection_points[current_city][other_boarder]:
                                 city_boarder = _city_boarder(node_positions[current_city], node_radius)
                                 current_track = connect_cities(rail_trans, grid_map, source, target, city_boarder)
-                                if target in outer_connection_points[current_city] and source in \
-                                    outer_connection_points[current_city] and len(through_path_cells[current_city]) < 1:
+                                if target in all_outer_connection_points and source in \
+                                    all_outer_connection_points and len(through_path_cells[current_city]) < 1:
                                     through_path_cells[current_city].extend(current_track)
                         else:
                             continue