All Chicago 10 PM newscasts led with the story. As Mike said the passenger was in his 70's (one report said 79) and shot through the window, the motive is unknown.
I would tend to doubt an argument with the conductor because that probably would have been observed by nearby passengers, and eye witnesses are not reporting any argument with the conductor and the shooter.
The train was 2 hours late, held for 4 hours in Naperville, and has arrived in Chicago, where other trains were being held so PAX could make their connections--so said the TV news reporters.