To whom does Whitman refer when he speaks of “My Captain,” and what “fearful trip” has been concluded in the poem "O Captain, My Captain"?

In his famous poem "O Captain, My Captain," Walt Whitman
is referring to Abraham Lincoln. The "fearful trip" which has just concluded is the
American Civil War.

Walt Whitman, who had lived in
Washington during most of the Civil War, was a great admirer of Abraham Lincoln. "O
Captain, My Captain" expresses poignantly the deep sense of grief and despair which
completely overshadowed the feeling of relief that the long war was finally over
when Lincoln was assassinated in its immediate aftermath. Published in 1865, a little
over six months after Lincoln's death, the poem became arguably Whitman's most popular
work, and frequently appears in anthologies. Current readers of the poem may remember
that it was used in the 1989 movie Dead Poets' Society, starring
Robin Williams.