About half way between West Egg and New York the motor road hastily joins the railroad and runs beside it for a quarter of a mile, so as to shrink away from a certain desolate area of land.
The narration establishes a sense of motion and emotion in the motor road and railroad. The railroad, the means of transportation favored by the wealthy, avoids the valley as if some kind of despicable filthiness will contaminate it. It thus seems to stand in for the perspective of the wealthy residents of the Eggs. The motor road “hastily joins” the railroad, with inharmonious abruptness, highlighting the difference between the Eggs and the valley.
On this map, the valley of ashes is right above Queens:
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