Baseline `num_trainstations` question

#1

When running the single agent training example from the baseline repo, there will be warnings popup from the command line:

/flatland-rl-path/flatland-rl/lib/python3.6/site-packages/flatland/envs/rail_generators.py:683: UserWarning: Could not set trainstations, please change initial parameters!!!!

The code file containts the initialization:

env = RailEnv(width=x_dim,
                  height=y_dim,
                  rail_generator=sparse_rail_generator(num_cities=5,
                                                       # Number of cities in map (where train stations are)
                                                       num_intersections=4,
                                                       # Number of intersections (no start / target)
                                                       num_trainstations=10,  # Number of possible start/targets on map
                                                       min_node_dist=3,  # Minimal distance of nodes                                                 
                                                       node_radius=2,  # Proximity of stations to city center
                                                       num_neighb=3,
                                                       # Number of connections to other cities/intersections
                                                       seed=15,  # Random seed
                                                       grid_mode=True,
                                                       enhance_intersection=False
                                                       ),
                  schedule_generator=sparse_schedule_generator(speed_ration_map),
                  number_of_agents=n_agents,
                  stochastic_data=stochastic_data,  # Malfunction data generator
                  obs_builder_object=TreeObservation)

in which num_cities = 5 but num_trainstations=10.

Are the warnings related to the settings?

If not, may I ask why trainstations# is more than cities#? From the doc, it seems trainstations can only exist in cities?

Thanks.