Emily Dickinson used a traditional ballad rhythm and meter that supports the content of the poem in "Because I could not stop for Death--," but she modified it with enjambment and caesura to add nuances of meaning.
Traditionally, ballads use iambic rhythm, meaning a pattern of alternating unstressed and stressed syllables. The length of lines in traditional ballads can vary from poem to poem, but two typical lengths are trimeter (six syllables) and tetrameter (eight syllables). Dickinson uses alternating tetrameter and trimeter, a form called a "fourteener" because there are fourteen feet (twenty-eight syllables) in each stanza.
The combination of iambic rhythm and short lines is perfect for ballads. Ballads tell a story, and this consistent, jaunty rhythm tends to move a story onward easily. It gives a feeling of walking briskly, or in this case, movement of the carriage with the wheels going around and around repetitively. This poem is a narrative--it tells a linear story about the person in the poem being picked up in a carriage by Death and Immortality, who then drive her to the graveyard. The ballad form supports the developing action of the story, and because many story poems have been written in this form, the reader can easily pick up on the feeling of a tale being told.
Dickinson was innovative in her use of enjambment and caesura to lend variety to the rhythm of her poetry. Enjambment refers to lines of poetry that do not have a hard stop at the end; caesura refers to hard stops in the middle of a line of poetry. Using enjambment creates a free flow of the language, while caesura creates a jerkiness. Both of these qualities--free movement and jerking--are typical of carriage rides and reinforce the content of the poem.
At the beginning, in stanza one, every line has a hard stop, indicated by a dash. There is no enjambment. This would be consistent with the carriage being at a stop and the jerky movements of being loaded into the carriage. The second stanza uses enjambment to show that the carriage is beginning to roll away. That smooth ride continues into the first line of the third stanza, but then it becomes jerky with hard stops at the end of every line and some caesura within lines. This may indicate that the ride is becoming less pleasant, which is matched by the air growing "quivering and chill." The final stanza returns to enjambment, consistent with the ongoing state of eternity spoken of in that stanza.
Dickinson was masterful in her use of the rhythm and meter of the traditional ballad to support the meaning of her poem and the addition of enjambment and caesura to suggest a more nuanced understanding of the progression of the story.