The hills step off into whiteness.
People or stars
Regard me sadly, I disappoint them.
The train leaves a line of breath.
O slow
Horse the color of rust,
Hooves, dolorous bells--
All morning the
This short, two word line is full of complex meaning. Firstly, there are the long, rhyming ‘O’ sounds. Although this is the shortest line in the poem the sounds are the slowest. There is a  contrast between the train with its smoke, signifying speed, and the reference to slowness. It would seem to be an  oxymoron . It is difficult to define why this train should be slow, unless Plath sees it as akin to the sluggish, dragging feeling of depression. 
 Although the sentence continues to the next line, the  enjambment  is hesitant. There is a pause after ‘slow’  - a  caesura  which separates it from the ‘horse’ in the following line. 
 We can speculate why Plath does this. Perhaps she is reluctant to think of horses. She once used to love horse-riding, as she describes in the poem  Ariel , but abandoned riding during her depressive episodes.
Sheep in Fog