The conversation between Jig and her American boyfriend sounds flat, tense, angry, and jaded. They keep repeating the same statements over and over and never seem to get anywhere. For example, the boyfriend tells Jig repeatedly that she should only have an abortion if she wants to. However, it is clear that the words he is saying are insincere and that he very much wants her to get rid of the baby.
She, in turn, wants to keep the baby and start building a relationship. The two are at impasse. They are stalled in their relationship, going nowhere, just as they are stuck at the train station waiting for a train. Hostility simmers just below the surface of what they say. Below the surface, both are bitterly unhappy.
Hemingway keeps the dialogue as lean and spare as possible and doesn't supply adverbs such as "angrily" or "bitterly" to guide the reader. The words the couple use are simple, and both Jig and her boyfriend rely on pronouns rather than referring specifically to what they mean. The man, for example, never uses the word "abortion," referring to it instead as "it."
Hemingway, as a modernist, is experimenting with communicating using as few words as possible, and he lets the repetition and the words themselves convey the anger and the tension the couple is experiencing. He uses very little narration once he sets the scene, letting the situation itself carry the emotion.
A example of the tension this couple is experiencing is exemplified in the following passage, in which the first speaker is Jig, the "it" being the baby she is pregnant with and the contemplated abortion:
"Doesn't it mean anything to you? We could get along." [Jig]
"Of course it does. But I don't want anybody but you. I don't want any one else. And I know it's perfectly simple." [the man]
"Yes, you know it's perfectly simple." [Jig]
"It's all right for you to say that, but I do know it." [the man]
"Would you do something for me now?" [Jig]
"I'd do anything for you." [the man]
"Would you please please please please please please please stop talking?" [Jig]
From the dialogue we hear Jig asking if "it," the baby, means anything to her boyfriend and saying they could make having a child work. The boyfriend says he only wants her, Jig, but returns immediately to what he really wants, the abortion. Obviously, he doesn't really care about Jig, or he would be more sensitive to her desire for the baby. Instead, he tells her "it" (the abortion) will be simple. Then, we can hear Jig throwing his words back in his face when she says "you know it's perfectly simple," meaning none of it is simple to her. We sense her hostility as well in the final sentence when she asks him to please stop talking, saying "please" seven times in a row, which conveys that she is very, very tired of having this same conversation.
Hemingway knows that the emotion underlying what the couple is saying is clear enough that he doesn't have to explain it to readers who are paying attention. But he is asking readers to pay attention to his words.