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:
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.