“A suddenness of trees” is a figurative way of implicitly indicating the motion of the train and the apparent backward movement of the trees. While sitting on a moving train, it seems all the objects outside are moving in the opposite direction. The “suddenness” of the train makes the objects appear as moving. Therefore, through this phrase, the emphasis is laid upon the speed of the train. Roethke gives readers an idea of how the external world appeared from the train.