From the Introduction: In other places a subway tunnel might have been dug for this line, but here the groundwater level is too high and the ground itself too soft. Thus the line was built as a much cheaper elevated railway. The numerous wide arches of this viaduct distributing the weight of the structure and the trains traversing on it. In addition streets can easily cross through any number of arches. Still most of the arches were boarded up and rented out as storage space or workshops.