Both "A Brook in the City" and "The Road Not Taken" draw on Frost's love of natural imagery. The brook ran through a meadow and a group, if not a full orchard, of apple trees when the person speaking in the poem first knew it. The source of the road runs through the undergrowth of a wooded area, then it branches into two paths at the location where the speaker is standing. In both poems, something is lost and that loss is mourned by the speaker.
Upon being forced to choose which road to follow, the speaker recognizes the loss of not being able to see where both lead. The hope of returning to take "The Road Not Taken" is tempered by awareness that "knowing how way leads on to way, I doubted if I should ever come back."
The brook, now covered over by the cinder and building materials of the city, was the site of adventure and wonder in years gone by for the speaker. The return is disturbing and disappointing because the brook has been banished, and wonders if its buried spirit haunts the city that has arisen above it.