I really appreciate the originality of this project, game has a nice core loop and it has its own identity. I like the little details in the map, creating the finished assets in 4 days is challenging, but you guys pulled it off. Music is nice too.
Some things I'd like to see are:
1. The attacking enemies as fully realized enemy models
2. Maybe some noise or feedback to indicate the train is starting to rev up.
3. Maybe a night battle? Could add some additional difficulty.
4. More train cars (weapons)
Other than that, great job guys. Keep up the great work!