I like to read the train station setting as a metaphorical parallel to the situation of the couple. The young woman in the story is pregnant and so is dealing with a "scheduled arrival". Unless something is done, the baby will come.
Similarly, a train will come into the station and when it does, the inevitable future will have become the present. Both characters will get on that train and ride to a predetermined destination. They would like to think that when they arrive there, it's still possible for them to be together and in a relationship, but they both seem to know that this is not the case. When they get off the train (i.e., when the baby is born), everything will be lost and they won't be able to stay together.
The couple has a chance to leave the station. They could avoid seeing the train, if they both wanted to turn away from that path. However, thier decision has already been made. They disagree about whether it is the right decision, as symbolized by the man crossing the tracks at the end of the story to separate himself from the woman, yet they will both board the train. The future is as inevitable as the arrival of the train in the station.
In the first part of Hemingway's narrative, there is the setting of white hills and the brown valley with "no shade" and "no trees," which are separated by "two lines of rails in the sun." Later in the story, Jig stands and walks to the end of the station where she can see "fields of grain and trees" a river, and mountains. And, across the field of grain, under "the shadow of a cloud," she sees the river through the trees.
Perhaps, then, the story can be interpreted in the context of the dialogue running parallel to the descriptions of setting: In the beginning, the conversation is about the abortion, and Jig considers the abortion, "Then I'll do it. Because I don't care about me." But, as the conversation continues, she reconsiders, "And once they take it away, you never get it back." She tells the man, "...we could have everything." Like the fertile fields of grain with trees of life, Jig looks to the other side and sees the river, symbolic of life. But, the man carries their bags back "around the station to the other tracks" where the brown ground and white hills lie.
When I read it, I was caught up in the idea of the railroad station as a place of transition, but also one that is limited as the track only goes in two directions even as she stares off into the mountains trying to make a decision. It suggests not only a place of change or transition but also some of the limits on the choices that can be made, in the end for her in some ways there are only two choices, keep the baby or don't.
Oh poster number 5... you have an interesting idea there. Freud would be proud. I agree with most above that the major symbolism is a place of transition- a point on a long journey. The fact that the story does not have a real resolution is also reinforced by the train tracks... we know their journey has not come to an end with this one discussion.
I'd considered the destination symbolism on first reading this, but now I think I perhaps should read it again. One of the posts has kind of addressed the concept of the hills representing the female. Is this because of the rounded curves, and this is where she wants to get to? Would the train, and the station then have a phallic symbolism?
Remember, too, that this story has both physical and mental/emotional destinations. Some critics read this story as if the girl has chosen an abortion (the hills, female parts, etc.) whereas the railroad station is the hiatus or "resting place" before getting to emotional, physical, and mental healing. Just food for thought...
Also, consider the symbolism of the railway station. It is a place of transition, not a destination in itself, but somewhere where you stop en route to where you are going. Metaphorically, where Jig and her partner are going, and if they are heading there together or separately, is unsure in the story, and thus the setting emphasises the decision that has to be taken and how there is still a "journey" yet to be made.
As I recall, the railroad station sits in a vast expanse of open land in Spain. On one side of the station, the land is barren. On the other side, the land is not barren. I think a stream cuts through it. This is the land that stretches away to the mountains in the distance. This is the side of the station where Jig stands to look at them, observing that they look like white elephants. There is symbolism at work in the setting. Jig wants a life beyond the empty relationship she has shared with the American. The barren side of the station seems to represent the kind of life she wants to give up. Jig does not want to end her pregnancy; she is identified with the life-filled side of the station. She appreciates the beauty of the hills; the American does not.