What do the train and the smoky, stuffy compartment represent?
The train with all the people in it might represent the collective sense of helplessness that envelops all people when a nation goes to war. There does not seem to be a lucid direction to the train and the people on it are not really focused on seeing the train to its natural end. It seems as if they are thrown together in a loose confederation of individuals who are bound by their common understanding that they are not fully in control of the conditions around them. They interact with one another in the train in order to find meaning, almost as if their dialogue is one that might clear up the smoky and stuffy conditions. This is something that is still obscured with the lack of resolution offered at the end of it.
To me, this is something of a gloomy story about how people are powerless in the face of what their governments want to do. They lead lives that are weak and oppressed -- they can't really have an impact. Things happen to them and they can't stop those things, no matter what they think of them. I think this is symbolized to some degree by the setting.
The people are on this old-style railroad car and it is just sitting there, going nowhere. It is a very stale atmosphere. It is oppressive and stuffy as you say. This shows how dull the people's lives are.
So the train and the inside of the car show the futility and powerlessness of the people's lives.