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There is chance that the initial direction of agents is not valid! For example, when agent.initial_position is a east-west rail, the agent.initial_direction may be NORTH sometimes, although the chance is very small.

Following script can reproduce the bug within several hundred of loops.

from flatland.envs.rail_env import RailEnv
from flatland.envs.rail_generators import SparseRailGen
import numpy as np

env = RailEnv(
    width=60,
    height=60,
    number_of_agents=4,
    rail_generator=SparseRailGen(max_num_cities=6, max_rails_between_cities=3),
)


cnt = 0
while True:
    env.reset()
    for agent in env.agents:
        possible_transitions = env.rail.get_transitions(
            *agent.initial_position, agent.initial_direction
        )
        if all(np.array(possible_transitions) == 0):
            print("Wrong initial direction!!!!! No possible transition from it!!!")
    cnt += 1
    print(f"\r{cnt}", end="")

My test environment:

• OS: Ubuntu 20.04 LTS (GNU/Linux 5.10.16.3-microsoft-standard-WSL2 x86_64)
• python==3.6.13
• flatland==3.0.15