I have not really studied the Philly map and am not familiar with PRR operations in the area, but typically the secondary yards are in place to serve local industry. Local crews were based out of these yards and would handle inbound and outbound traffic.
Ed
lots of industries, warehouses, and piers got (get?) one or two cars a day and/or ship one or two cars a day. Runniin one and two car trains crosstown to the road-train classification yards would not be economical and would have led to congestion. So the regional yards consilidate outgoing movements and switched-out incoming movements of 20-50 to and from the main yards.
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