In the poem "Metaphors," what are Plath's feelings when she says she has "boarded the train there's no getting off"?

Famously this poem talks about Plath's own feelings when she was
pregnant, and she uses a series of metaphors to describe herself. However, within these
descriptions we can see her own feelings about her situation and her life. Descriptions
such as "I'm a means, a stage, a cow in calf" really indicate her sense of loss of her
own identity and the way that, in a sense, pregnancy has dehumanised her and she has
lost something of herself. The culmination of this poem comes in the final line that you
have identified, that clearly shows that whatever Plath feels about what is happening to
her, it is too late to do anything about it now, for she has:
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Boarded the train there's no getting
off.

This clearly displays the feelings
of Plath in recognising that this is a process that has robbed her of control and which
is now inevitable. It conveys the negative feelings that she has when considering her
pregnancy and expresses a sense of powerlessness and
helplessness.