To Ridgely Torrence
On Last Looking into His 'Hesperides'
I often see flowers from a passing car
That are gone before I can tell what they are.
I want to get out of the train and go back
To see what they were beside the track.
I name all the flowers I am sure they weren't;
“Car” refers not an automobile, but to a train car. The narrator is riding a train. 
 The laws of physics provide this simple scene. He watches flowers along the track. Since the train is moving so fast, they pass by quickly. 
 Metaphorically, the “car” could represent a coffin. During a funeral, flowers are often presented to the deceased body. Obviously, a dead person cannot tell what the flowers literally are.
A Passing Glimpse