I felt a Funeral, in my Brain, (line 1)
The most curious word in this line is "felt." We might expect "imagined" or "dreamt" instead. The poem certainly reads a lot like a visual dream, with its frequent and sudden shifts in perspective. But the funeral could also be a metaphor for some conscious feeling or emotion. Whatever kind of reality we're talking about, this poem takes us deep, deep into the mind.
That Sense was breaking through– (line 4)
What is her sense "breaking through"? Are these her natural resistances to feeling? Are they mental resistances to some memory she has avoided thinking about? Dickinson is famous for leaving out important words or information. Her poems are incredibly condensed. We think she wants us to fill in the gaps with whatever comes into our imaginations.
And creak across my Soul With those same Boots of Lead, again, Then Space–began to toll, (lines 10-12)
Dickinson's poems often feature some dreamlike space. She spent a lot of her life writing inside her room, and rooms and other closed spaces could seem like an entire universe to her. The sudden shift between the place where the funeral takes place and all of "Space" suggests a dream.