Personally, as a symbol in the short story "The Utterly Perfect Murder," I like the train. To me, it is a symbol of Doug's journey into his past. This is actually supported by the text when Bradbury says: "The train moved. My wife was gone. I rode off into the past."
Also, more indirectly, reread the scenes of him riding on the train. Notice the way that the trains movements across the country coincide with Doug's recollections of his past. As it goes through Kansas City (and a "beaut of a thunderstorm") he thinks about how Ralph had liked to punch him in the arm. Later, as the "night country rolled by" he thinks of being pushed into the mud in his good clothes. Then later (as the rain beats on the train windows) he thinks of how one-sided the friendship was. By the train stops, so have his recollections and he has arrived in his past:
"They say you can't go home again...That is a lie. If you are lucky and time it right, you arrive at sunset when the old town is filled with yellow light."
So, for me it is the train that is a great symbol in the story. There are other possibilities, but I think this one is most poignant.