The current Allentown yard is a much different facility than it was prior to being rebuilt in 1978-9. Today, the main tracks are next to the Lehigh canal, on the opposite side of the yard from this camera vantage point. The single hump is off to the left. In this view, a set of LV C628's is making a reverse move on the eastbound main as it travels from the engine terminal in Bethlehem to the west end of the yard to get its westbound train. The main tracks run between the two hump yards - The heavy side (eastbound) yard is beyond the power, and the light side (westbound) yard is closer to the camera. I don't recall ever seeing it documented, but it seems logical that the naming convention was related to the flow of anthracite hopper cars - empties west, loads east. Today's single hump yard is an expansion of the light side yard. The heavy side hump and its supporting structures were bulldozed into oblivion.
