A consortium led by U.S.-based project management and construction company Bechtel has begun tunneling on the first phase of Saudi Arabia’s first underground rail system.

The consortium, which includes Saudi company, Almabani General Contractors, Middle East-based Consolidated Contractors Company, and Germany’sSiemens AG, is responsible for the$10 billion contract for the design, construction, train cars, signaling, electrification and integration of Lines 1 and 2.

The work includes 39 stations, two of which are key interchange stations – the Olaya Station, situated in the center of the Saudi capital of Riyadh at the intersection of Lines 1 and 2, and the King Abdullah Financial District Station, located slightly to the north on Line 1.

According to California-headquartered Bechtel, the first tunnel boring machine will steadily ramp up to its planned average tunneling rate of about 325 feet per week and is expected to complete its journey by mid-2016. In total, seven tunnel boring machines will be deployed to dig and construct more than 21 miles of tunnels beneath the capital city.

Riyadh is one of the world’s fastest-growing cities, with a population expected to increase 50 percent by 2035 to 7.5 million.

The Riyadh Metro is part of a 25-year strategic plan prepared by the High Commission for the Development of Arriyadh to cater for this growth. When complete, the 109-mile, six-line driverless network will serve 400,000 passengers and be one of the largest metropolitan transit systems in the world.