Complex sentence: A new surface line across the center of town was politically impossible and prohibitively expensive; building around the outskirts of town would have required massive regrading and bridging, as the terrain is extremely hilly and the line would cut across every watershed flowing into the harbor.
Split into simpler sentences: A new surface line across the center of town was politically impossible and prohibitively expensive. Building around the outskirts of town would have required massive regrading and bridging, as the terrain is extremely hilly and the line would cut across every watershed flowing into the harbor.