The speaker is sending his adolescent son off to his mother's home. The boy carries with him a "black duffle bag" (line 7) and he wants to tell his son that he is "entering into the light." There is a bittersweetness in both of these lines. The bag can stand metaphorically for the secrets of his son's inner life that the father will never know, or it can represent the death of their relationship. Likewise, entering the light might be positive, as he is literally and physically becoming enlightened by maturing, or, negatively, it may be another reference to the death of the relationship that they have known, like the common reporting of people who have had near-death experiences who say they "entered a light" (line 11). The "tinted windows" are then contrasted with the "dazzle" of the Hudson. The father worries that the boy is not mature enough to see through either one.
There are more things for the father to worry about. He pictures the train entering a tunnel. The tunnel may be a Freudian reference to the boy's awakening sexuality. The point of all this "either-or" is that the father does have mixed feelings about his child leaving him an becoming an adult, feelings many parents and children can identify with.