What is the deeper meaning of the following poem?

"subway"

i thought that if he could stoop
to pick out rubbish, each piece
placed in his bag--a tedious job
in front of crowds, all day
the trains at a steady roar,
the lighting dim, the air stagnant--
from bin to bin, searching
to the bottom for gum wrappers,
crumpled newspapers, torn sandwich
bags, cigarette stubs, particles
clinging to his fingers. all this
without a word, bending
at the foot of a steel pillar,
it was not too much for me
to be witness.