FWIW, Crossrail is on this list of the 25 "most impressive" projects - http://www.popularmechanics.com/technology/g2121/the-worlds-... .
https://en.wikipedia.org/wiki/List_of_transport_megaprojects describes it as a "megaproject", and I can agree that everything which is a megaproject is one of the largest construction projects in the world.
Looking at the list of projects which are "under construction", and ordered by cost, Crossrail is #6 at US$25B and the GBT is #10 at US$10B.