fixed minimal city size

examples/flatland_2_0_example.py

@@ -37,7 +37,7 @@ env = RailEnv(width=50,
                                                    seed=1,  # Random seed
                                                    grid_mode=False,
                                                    max_rails_between_cities=3,
-                                                   max_rails_in_city=6,
+                                                   max_rails_in_city=2,
                                                    ),
               schedule_generator=sparse_schedule_generator(speed_ration_map),
               number_of_agents=10,

flatland/envs/rail_generators.py

@@ -558,7 +558,7 @@ def sparse_rail_generator(max_num_cities: int = 5, grid_mode: bool = False, max_
 
         rail_trans = RailEnvTransitions()
         grid_map = GridTransitionMap(width=width, height=height, transitions=rail_trans)
-        city_radius = int(np.ceil((max_rails_in_city + 2) / 2.0)) + 1
+        city_radius = int(np.ceil((max_rails_in_city + 2) / 2.0)) + 2
         vector_field = np.zeros(shape=(height, width)) - 1.
 
         min_nr_rails_in_city = 3