I can definitely see your issue with this poem. I too was perplexed at the idea of trying to paraphrase the poem without making myself an observer.
I think you have to look at the last lines and use them to put the situation together. This woman is beaten down. Her little girl has a broken arm and "She doesn't like to talk/ about that," illustrating possibly, that it was broken for her, probably by the same hand that is attached to the "arm raised over them,/ Its motion [beginning] like a blessing."
This is the story of a battered woman and her two abused children. It's not about the subway ride and the people observing her. Rather it's about the silence that the woman has committed herself to when she is not near her abuser and when she's in the presence of people that could probably help her. Her's is a life that represents many lives of women and children. The poem may be set in a subway car, but in reality, I think it's set anywhere there is a woman hiding from abuse and having to remain silent.