Our speaker pretends to be all mysterious and sphinxlike, but really spells out the riddle to give us an easy solution: the "I" in this poem is a pregnant woman, who isn't exactly reveling in the expectant mother glow.
Sure, she's telling us, the readers, that she's a riddle, but we think that she finds herself to be a riddle as well. She seems pretty detached from this so-called loaf of bread growing in her belly, as if she's grasping at these metaphors to understand just what it is that has happened to her. And she's certainly not happy about it.
Why? Well sometimes she feels she's just a means to an end, like she's being used as someone who bears a child and nothing more. She also feels trapped—stuck on a train she can never get off of. As unfortunate as these emotions sound, we can bet that many an expectant mother has felt them, too. After all, having a kid is a Big Deal, and once you have that kid, there really is no turning back.