"Well, I think the questioner wants the truth. The doubter wants to be told there ain't no such thing."
The black man calls himself a questioner of religion instead of doubter. It is clear that he believes, and that the Bible means a lot to him, but he still doesn't run from questioning the validity of things. He wants to get through to the truth, while a person doubting wants confirmation of their doubts, rather than questioning what is the truth and what not.
"Depends on what kind of night we talkin bout."
The black man tells the white professor that he doesn't want him going out into the night. The kind of night he talks about here is the death or suicide. He desperately wants to save this man's life and keep him there until he finds the right words to prevent him from jumping in front of "The Sunset Limited".
"So he's a different kind of commuter. He's worlds away from them everyday travelers."
The black man is trying to make the white see the truth that everyone might be carrying the similar suffering inside them and that his fellow travelers might not be so different from him. It is clear that the professor set his mind onto being a unique specimen who discovered the sufferings of the world and wants to escape from it, he sees others just as brainless pawns. He demonstrated sociopath behavior and at the end the black man will discover that there is no right words to say to change this sort of person's mind.
