Complex sentence: Originally, the Freeport Diversion ended at an at-grade Y-junction with King Street East, with traffic from the Freeport Diversion treated as the mainline traffic at that interchange.
Split into simpler sentences: Originally, the Freeport Diversion ended at an at-grade Y-junction with King Street East. Traffic from the Freeport Diversion was treated as the mainline traffic at that interchange.