I would say that the speaker of this poem has a negative attitude toward her pregnancy. Typically, a woman who calls herself, or is called, an "elephant" or is said to be as big as a "house" is not receiving a compliment. Further, and perhaps more tellingly, the speaker says that "Money's new-minted in this fat purse." In this metaphor, the baby is the thing of value, the money, and she is simply the "fat purse" that contains the valuable item. Again, "fat" is also not a word that has a positive connotation: in this situation, used to describe a full purse, "fat" takes on a bit of positivity, but only in relation to the fact that the purse is fat because it contains so much of something so valuable. It is the money that it important, not the purse.
Also, we might describe something as "a means to an end," and the end is the important thing. However, here, the speaker is simply the "means," and we can assume that the baby is the end. The baby is, thus, the important thing; the mother, less so. Finally, the inevitability of the final metaphor which compares pregnancy to a train which one cannot get off makes it seem as though, if the speaker had a choice, she would choose to get off that train. All this appears to confirm the speaker's negative attitude toward her pregnancy.