In Boyne's The Boy in the Striped Pajamas, what is Bruno's reasoning for not saying anything to the Jews on the crowded train?

Bruno's reasoning behind his silence towards the Jewish people on the crowded train car can be found in Chapter 5 of The Boy in the Striped Pajamas.

For a moment he considered running across the platform to tell the people about the empty seats in his carriage, but he decided not to as something told him that if it didn't make Mother angry, it would probably make Gretel furious, and that would be worse still.

Therefore, Bruno's reasoning is based upon the fact that both his mother and his sister would be angry with him.  This reasoning is very much a nine-year-old boy's reasoning.  Bruno's surprise is based upon the fact that the two trains are obviously running in the same direction.  They are both going to the same area.  Bruno does not understand why the people would be shoved tightly into boxcars instead of seated in comfortable seats in passenger cars.  The answer can be found in the horror of the situation:  the people in the boxcars are Jewish and the people in the comfortable cars are not.  As a nine-year-old boy, Bruno is unaware of this fact.