Sandburg’s “Picnic Boat” is a snapshot of the aftermath of a Sunday picnic. It’s a visual and musical image picnicker returning at the end of a long leisurely day.

Sunday night and the park policemen tell each other it
is dark as a stack of black cats on Lake Michigan.
A big picnic boat comes home to Chicago from the peach
farms of Saugatuck.
Hundreds of electric bulbs break the night’s darkness, a
flock of red and yellow birds with wings at a standstill.
Running along the deck railings are festoons and leaping
in curves are loops of light from prow and stern
to the tall smokestacks.
Over the hoarse crunch of waves at my pier comes a
hoarse answer in the rhythmic oompa of the brasses
playing a Polish folk-song for the home-comers.

Sandburg must have been aware that in July 1915, the steamship Eastland capsized just after leaving the pier in Chicago, killing 844 picnickers. Also, see the 1906 General Slocum picnic disaster in New York City. 

See Sandburg, Carl. “Picnic Boat.” In Chicago Poems. New York: Henry Holt and Company, 1916.