I have been one acquainted with the night.
I have walked out in rain—and back in rain.
I have outwalked the furthest city light.
I have looked down the saddest city lane.
I have passed by the watchman on his beat
The “rain” could be a metaphor for his constant feelings of depression and loneliness. He starts out his walks feeling the same way he ends his walks, hence he has “walked out in rain-and back in rain.” Nothing changes for the narrator on the walks; his depression stays constant.
Acquainted with the Night