The Carriage held but just Ourselves –
The “Carriage” seems to be a hearse, which held the dead speaker was shared with Death, but also with Immortality by this Dickinson intimates that death is a necessary part of the passage to immortality.
Although this is probably not what she meant, Dickinson and other great writers are ‘immortal’ despite death because their words live on and are read centuries later.
Jay-Z says the same thing in Threat.
I may be wrong but I think a grave isn’t involved at this point yet.
They’re about to travel to the grave — depicted in stanza 5 as a “house”.
At this point it much more looks like a funeral procession or the “your life flashes before your eyes”-topos (not sure if this was already a thing in the 19. century)
To help improve the meaning of these lyrics, visit “Because I could not stop for Death (712)” by Emily Dickinson and leave a comment on the lyrics box