I had fun playing this. It even played pretty well on a laptop at medium settings.
While it seemed like there's still stuff to do, it was a nice little zombie survival game. I expect you plan to still work on this one as it felt like there was a lot more you wanted to do with it.
For myself, I felt that as the waves got bigger that the carriage felt a bit small for the gameplay style, there wasn't really enough space to dodge enemies coming from both ends. But this could be balanced depending on how you progress the game. I could see that if you keep the environment small, I think there should be a greater variety of weapons or ways to defend your self, if you want to keep the weapons limited, I think the environment should be expanded (this could be by visiting underground/metro stations to keep with the train theme, where bigger waves happen on the platforms while the train rides between could have smaller waves or bosses).