Who saves Russell in the end of the book and why ?

In the book "Maniac Magee" is invited to a birthday party by one of his white friends the McNabs.  He decides to attend the party and asks the boys if he can bring one of his friends.  He is told that he can.  Maniac thinks about how blacks and whites do not go into each others homes or streets.  He understands that their lack of knowledge about one another has affected their ability to get long with each other.  Maniac invites Mars Bar, the black boy who does not like Maniac very much.  Finally, after much coaxing Mars Bar agrees to go to the party with Magee.

Mars Bar is uncomfortable at first but he joins the kids in their games.  Russell is fascinated by him.  He yells to play the game Rebels which one side plays whites and the others blacks.  It is clear that in the kid’s version the blacks always lose.  Mars Bar becomes frustrate by the kid’s behavior and leaves with Maniac following him.

Maniac returns to sleeping wherever he can find a place.  He runs each day but not on the trestle where his parents had died.  One day he finds himself running alongside Mars Bar.  They run together for awhile.  They continue running together in the mornings. 

The boys were called buy Piper for help. Russell had been playing a game on the trestle.  The child had not realized how high up he was but now he was frozen in fear.  The P&W trolley was only 20 feet away.  Magee could not move. ting back on his parent's death, he finally runs away. 

Mars Bar rescued Russell by getting him off the trestle just in time before the trolley could hit him because it was the right thing to do.  Maniac was unable to do it.