Is tunneling really the issue? The tunnel for the extension of the line 14 in Paris was completed last week, at a speed of 25 meters by day (twice as fast as expected). Yet the line won't open until at least 2020, many years after the initial deadline. Construction was stopped for more than a year because of groundwater flooding in one of the new stations, which prevented the boring machine from going through. I'm not sure the tunnel boring itself has ever been a technical challenge.