 In the story, the System had existed for centuries before the Tube, and it is mentioned in the second Appendix that the beginning of construction of the Tube caused the Undergrounders considerable alarm, although tempered by pride that the Abovegrounders were (seemingly) imitating them for a change.The key clue to the location of the System is the River Fleet, diverted underground in the eighteenth century.It serves as the spine of the canal network and the setting for one of the most dramatic scenes in the book, as both the train (with Widgie on board) and the boat (with Jen) race against time to reach and pass a bridge over it as the river becomes swollen and flooded.