Complex sentence: North-bound local trains were often held up to avoid delaying expresses, and then made up time over the bridge; the gradient onto the bridge at the north prevented similar high speeds on south-bound locals.
Split into simpler sentences: North-bound local trains were often held up to avoid delaying expresses, and then made up time while travelling over the bridge. The gradient onto the bridge at the northern end prevented similar high speeds on south-bound locals.