Nick reflects on his train ride to work that morning. Why did he sit on the opposite side of the railroad car as it passed the ash-heaps?

From the text:

"When I passed the ash-heaps on the train that morning I had crossed deliberately to the other side of the car. I suppose there’d be a curious crowd around there all day with little boys searching for dark spots in the dust, and some garrulous man telling over and over what had happened, until it became less and less real even to him and he could tell it no longer, and Myrtle Wilson’s tragic achievement was forgotten."