This incident comes at the end of Chapter 2, when Jacob, aimless and distraught over his parents' death and the loss of their house, decides on a whim to jump on a train and see where it takes him. We need to remember the setting of the novel. It occurs during the Great Depression in America, where thousands of people were destitute and out of work. The reason therefore why the men in the carriage treat Jacob with animosity is because they think he is a bum who is trying to get a free ride or steal something like so many people did at this time of history.
This is why Blackie, the largest of the men in the carriage, tries to throw him off the train straight away, even though it is moving. The Great Depression meant that Jacob was just one of many, many individuals who had nowhere to go and were seeking their fortune in other parts of America. Blackie did not want this to be at the expense of the Benzini Brothers and their own security. After all, with so many in want, they could hardly take in all the waifs and strays that there were.