Here are some environment/prop concepts that I did for ALKA: Automated Letal Kinetic Assistance, 4 players coop shooter game in development at NAD-UQAC. While I started with general thumbnails for the train station and the boss arena, I later focused on specific elements of the train station only because most of it would end up being done with reused assets and modular kits from the past levels. The train station contains a train that is an important part of the gameplay.
There were a lot of technical and artistic limitations due to the online multiplayer aspect, which was very challenging, but it was fun to seeing the train level take shape.