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;
Not fireweed loving where woods have burnt--
Not bluebells gracing a tunnel mouth--
He wants to get out and observe the flowers more carefully. 
 Since the train is moving, getting out would be extremely risky; this notion of jumping off leads to the  final lines, in which he talks about Heaven (death). 
 There’s also a continuation the funeral metaphor: the train is leading to his final destination.
A Passing Glimpse