I like the main gameplay loop of shooting and building rails and turrets. If you flesh it out I think it has a lot of potential.
One suggestion I have is to make the train come after a while. If it reaches the end of the tracks before they are finished you lose, or if gets shot at by the enemies you also lose. This would force players to take more risks to get resources to finish the tracks quickly and also incentivize building turrets along the tracks to protect the train.
Also, great use of the art assets.