This is what Mars Bar wants to know at the end of the story. Maniac's fearlessness has reached mythic proportion, so it's quite a mystery why he didn't rescue Russell from the train trestle. As Mars Bar says, "Listen, man, I know you wasn't scared. I know it."
Remember how the story begins, with the death of Maniac's parents in a tragic trolley car accident. The accident occurred on the very same trestle Russell has wandered out on, and in that accident, the trolley car plunged off the bridge into the river. Because of those terrible memories, Maniac has avoided that place for years. He confides in Mars Bar that when he saw the trestle, it made "the nightmare worse than ever." So he ran, just as he ran most of his childhood.
He could but he wouldn't because he thought of his parents.