Complex sentence: But not only was the route long, it was slow, as curves on the Warren Railroad section, which constituted about a third of the route's 39 miles (63 km), restricted trains to 50 mph (80 km/h).
Split into simpler sentences: But not only was the route long, it was slow. Curves on the Warren Railroad section, which constituted about a third of the route's 39 miles (63 km), restricted trains to 50 mph (80 km/h).