"Hills like White Elephants" is about one complex thing primarily - a relationship that has become tense, complicated and distressed by pregnancy. However, as much as "Hills like White Elephants" is about pregnancy, it is also about how two people deal with the prospect of pregnancy.
The man suggests an abortion and the woman worries that this will ruin their relationship. The disagreement between the two characters as to what the best path of action might be is dramatized by the position of the characters at the end of the story with the man standing alone on the far side of the train tracks with the couple's baggage.
The division and disagreement over what to do about the pregnancy is the primary conflict and subject of the story, but alongside this conflict is a sentimental agreement. Both characters would like things to go back to being the way they once were, before the pregnancy. Something has been taken from both of the characters which each treasured and enjoyed. Sitting in the station, waiting for the train (and the scheduled/inevitable future), the couple share a significant regret.