I think you should add wheels to your train, add vents on the top of the train, add some NPCs ,Add a front where the engineer and operator work. Add some colors because it looks boring and add a back door with a metal deck
Maybe you could add more exterior design, as well as interior. It looks very simple, but if that is your aim, then you don’t need to. It also looks very blocky.
I really like the exterior of the train it looks really nice. The only problem with it I see are that the seats are too simple. I think you should give the passengers some armrests and put some space between the seats so they arent sitting on top of each other.
You’re doing good so far. Might I suggest adding textures to the interior of the train and adjusting the lighting as well to add detail. Blender is also something you should look into, it can help a lot to make your creations look more detailed and realistic.
Maybe try to fill empty space inside by seats? Also you can try to detalize seats a bit and add lightings inside. Anyway goodluck working on your game!