What is the symbolic meaning of trains in Murray's Train Whistle Guitar?

Train Whistle Guitar is set in the 1920s and is a semi-autobiographical novel of a young boy growing up in Alabama. The novel was written by Albert Murray and published in the year 1974. The title refers to the sound a guitar can make when someone is playing the blues, but the train imagery is present throughout the entire book. Those sounds are evocative for the protagonist, allowing him to remember with great fondness his childhood in Alabama. His feelings and remembrances become more important than the actual place itself. According to Murray, Gasoline Point became

more of a location in time than an intersection on a map.

Music is vastly important in this novel; in fact, it could be argued that it underlies the entire story. The novel itself is written in an almost staccato-type fashion, reminiscent of jazz. Scooter is the main character, and he darts in and out of the narrative as Murray introduces other characters. We witness terrible scenes—such as when a woman in a boardinghouse stabs her coarse, cheating husband—and moments of joy, during which we see Scooter's contagious enthusiasm for everyday life.

In Gasoline Point, everyone knows everyone and there are frequent neighborhood gatherings. At most events, there is storytelling, which is a time-honored custom. Most people understand that the storytellers are prone to exaggeration, but no one questions it, because that is part of the fun. Although the inhabitants of Gasoline Point are poverty-stricken, they are very close to each other and are dependent upon one another. Scooter's mother reminds him,





Don’t matter a bit of difference in the world where you come from you still got to do the best you can with what you come here with. Don’t care if it ain’t nothing else but just your health and strength, you better be thankful for that instead of going around trying to make out like you born with some kind of silver spoon in your mouth.

There are very few white characters in the novel, with the exception of a white policeman who raids an African American music club and threatens to close it down. In doing so, he kicks the keys off the piano, damaging it beyond repair. For the people of Gasoline Point, this is an unforgivable sin; they cannot live without music. The police officer is killed and his body hidden somewhere it will never be found.
In one of the book's earlier passages, Scooter is talking to a friend about Luzana Cholly, who is something of a local hero to the boys of Gasoline Point. Scooter is waxing eloquent about the colors of his childhood home, and he says that Luzana has a steel blue hue about him that reminds Scooter of a train; Luzana also carries a blue steel gun in his holster. But mostly, Scooter says, blue steel is the color you remember when you remember how his guitar sounds when he plays it. So the sound of the train is completely interwoven with music, and music is interwoven with everything in Gasoline Point.
http://facpub.stjohns.edu/~ganterg/sjureview/vol1-1/murray.html