Complex sentence: The Spey Bridge was unfinished when the line opened, so initially passengers walked across the adjacent road bridge as the locomotive was detached and crossed before the carriages were hauled over by ropes.
Split into simpler sentences: The Spey Bridge was unfinished when the line opened, so initially passengers disembarked and walked across the adjacent road bridge. The locomotive was detached and crossed before the carriages were hauled over by ropes.