Welcome to one of Europe’s biggest infrastructure projects: an ambitious scheme to encircle Paris with a new metro loop, with 68 new stations, it will cost nearly €40bn.

The new station at La Défense, built as part of the westward extension of the e line, will link up to a huge looping network known as the “Grand Paris Express”. Most of the French capital’s existing rail and metro lines are there to carry people in and out of the city centre. The new underground loops, by contrast, focus on moving them around the suburbs...