There are two main symbols that really stick out to me. The first is the Red Convertible, and the other is the water at the very end of the story.
I think that the convertible symbolizes the relationship between Marty and Stephan. When they first buy it, it is brand new, and so is their relationship. They are on very good terms, and they are two very happy brothers. However, when they return from going to the many places that they do, Stephan goes to the army, putting their relationship on hold. The car is also put away while Stephan is gone. Marty takes care of the car very well, and keeps it in very good condition. Marty also thinks of Stephan a lot while he is gone. .
However, when Stephan returns, Marty describes Stephan as a changed person. Marty clearly tells the reader that he doesn't like the change in Stephan. When he "whacks up" the car, it shows that Marty doesn't want to have a relationship with the Stephan that has just come home. However, Stephan finds the car, and even though he knows that Marty had just wrecked it, he yells at Marty for not keeping the car in top condition. Everyday Stephan would go outside and fix up the car. Even when it was really cold outside. This shows that Stephan knows that he has changed, and that he doesn't want to end the relationship that he once had with his brother. He wants it back, and he tries to show it by fixing up the car again. When they go for a drive, they aren't nearly as uncomfortable as they were when Stephan first got home, and everything seems to be going better. But, Stephan jumps in the river at the very end, and Marty, seeing this, lets the car go into the river too. This shows that Marty lost both his brother and the car. He can't have a relationship with his brother if he is dead, so he doesn't want the car anymore either. .
The second symbol is the water in the river. To me, that symbolized life itself. There is a strong current that pulls Stephan away, and it's basically saying that Stephan was so overwhelmed with post-war affects, and he just couldn't handle life anymore, so he just gives in, and lets the water (life) carry him away defeated.