train sound (CC0): https://freesound.org/people/SGAK/sounds/467839/
factory sound (CC BY 3.0): https://freesound.org/people/The%20Baron/sounds/98444/
collector sound (CC BY 3.0): https://freesound.org/people/The%20Baron/sounds/98435/
crowd sound (CC0): https://freesound.org/people/sonically_sound/sounds/625112/