couple may be the only passengers boarding here. I believe it is only a way of portraying the setting, although it also suggests the time pressure. They don't have to do anything during the forty minutes the train is approaching, but once it stops they will only have two minutes to decide to get aboard or stay where they are.

In “Hills Like White Elephants, the unnamed American is having a serious discussion with his girlfriend Jig about whether she should have an abortion. This was a dangerous and illegal operation in the period, and the man wants her to go through with it. She does not want an abortion, and is possibly more interested in getting married or at least having some assurance that she will be cared for if she undergoes it. This is a major and serious relationship discussion, and, given how many women died or were permanently injured from illegal abortions, a major life and health crisis for Jig. Having an illegitimate baby in that time would not have been an easy choice either.

Confronted with this sort of huge dilemma for Jig, the mention that the train leaves in 40 minutes and only stops for two suggests that the man is, to put it really crassly, a major jerk. It limits the depth of the discussion and suggests that he’s not willing to invest time in being supportive. Rather than consider that he is the father of the baby and has responsibilities, it suggests that he wants to deal with the pregnancy merely as a minor annoyance that can be handled quickly and superficially.

I don't see the time factor as immediately relevant in terms of the plot.  Yet, I think that it is significant in terms of what Hemingway sets out to do in the short story.  One of the reasons why this short story is so powerful is that it really shows an example of Hemingway wanting to recreate "a moment," an instant in the life of two people.  He does this with a recreation of everything, a sense that he is the author but also one that prevents him from coloring this recreation with his own bias.  He wants to use language to recreate "everything" about that moment, that instant where the couple endure this instant that happens in real time, but is one whose effect will be lasting.  It is for this reason that Hemingway describes the situation with strong detail, and yet does not use this to construct something more elaborate.  He is presenting reality, in all of its complexity and simultaneous simplicity.  This style is one where time has to be recognized.  Time is not used as a symbol or something to represent more than what it is.  It is the undercurrent of the moment, the ticking reality that encompasses both the man and Jig.  The train comes, stops, and moves on.  The couple has to deal with these implications as their own relationship progresses.  The choices that they have to make regarding Jig's pregnancy is a part of this.  They simply cannot wish to remain "still," as time continually moves on.  This recreation of the instant is made more meaningful with the presence of time, and Hemingway's use of it helps to bring out the idea that we might wish otherwise, but we are both bound by our actions and the construction of time which is inescapable.  I think that this becomes the reason why time is important in the short story and in Hemingway's construction of it.