In Chapter 26, what is the significance of Tom's tale of the convict?
Tom is relating a story about a man he knew in prison that went crazy. This guy tried to escape a bunch of times but always told people about it, so he was never successful. After the last time he got caught, the other prisoners laughed at him until he killed himself. Tom's point was that people who are confined can do some crazy things.
We can relate this to the Joad's and all the Okie's in general. Although they aren't in prison, they are confined to their current lifestyle and forced to follow the direction of police and bosses that they don't like - a stark contrast to the life they enjoyed on their own farms. Although Tom doesn't come out and say it, we can infer that some of the Okie's will eventually get tired of how they are treated and do something crazy. We see actual evidence of this later in the chapter when Tom finds Casy again and kills another man.