Bishop’s fascination with the "third rail" stems from a set of private associations the poet refuses to develop within the poem. These associations are preserved, instead, in her working notebooks, where a direct relationship is forged between the fatal third rail and the dangers of alcoholism. In the journal she kept following her graduation from Vassar, Bishop plays with certain ideas that would eventually find their way into "the Man-Moth," including the observation that "the third rail is almost worth some sort of prose poem. Running along silently, as insincere as poison" ("Recorded Observations," 1934-1976, p. 6). As we have already seen, she would come to think of alochol as a poison with "flattering" effects, thus establishing a connection between ingratiating liquor and the "insincere" third rail that runs beside the subway track like an "unbroken draught of poison." Alternating between a tone of elevation and deflation (to match the ascending and descending fortunes of her protagonist), Bishop creates in her hyphenated creature a veiled portrait of the artist as addict.
Underscoring this theme of consumption and addiction is the man-moth’s resemblance to the Baudelarian vampire, imbiber of forbidden fluids. … Like the vampire the man-moth seeks to penetrate the physical boundaries of this universe and be born into a new existence, one that escapes the laws of mortality and gravity that weigh down the natural man. Thinking of the moon "as a small hole at the top of the sky," his aim is to "push his small head through that round clean opening." But instead of escaping those laws he is trapped in an existence ruled by bodily drives and marked by reiteration (he "must be carried through artificial tunnels and dream recurrent dreams").
Along with the inference of sexual deviance or unnaturalness that vampirism inevitably suggests, Bishop’s allusions to the undead reinforce the impression that she is grappling in this poem with the compulsive artist, a slave to the body’s mechanisms and to the machinery of art.
from Marilyn May Lombardi, "Abnormal Thirst: Addiction and the Poéte Maudit," Chapter 5 in The Body and the Song: Elizabeth Bishop’s Poetics (Carbondale: Southern Illinois University Press, 1992)