Riyadh, the capital of Saudi Arabia, boasts a population of over 6 million. But as of now, it has no public-transportation system.

But that's about to change. By the end of 2018, the city is expected to open the biggest urban mass-transit system that's ever been created from scratch.

The new network, currently under construction, will be comprised of six metro lines that connect 85 stations and span nearly 110 miles of track. A new bus system is being built and implemented simultaneously.

Saudi officials approved the plan in 2012 after population-growth forecasts for Riyadh suggested a 50% increase by 2035.

To create the system, the Saudi government awarded contracts to a variety of contracting and engineering companies from all over the world. The largest contract, worth a whopping $10 billion, was given to Bechtel, a US-based firm specializing in mega-projects.

You may recognize its work from England's Channel Tunnel, the Bay Area's BART system, or Athens' Metro. According to Fortune, Bechtel's Riyadh project is the biggest lump-sum civil-engineering contract that's ever been given to a single team.

The company, with the help of a consortium that includes Almabani General Contractors, the Consolidated Contractors Co., and Siemens, is in charge of the two most complicated lines of the system's six. Together, the group is responsible for the entire design — construction process, cars, signals, wiring and all. The nearly 40 miles of track are currently being built underneath central Riyadh and include 39 stations.

Men at the construction site of the Riyadh Metro on May 4. Faisal Nasser/Reuters To build the system, Bechtel is using powerful tunnel-boring machines, which weigh 1,000 tons and are called Mneefah, after the horse ridden by Saudi Arabia's founding king. Each machine can tunnel through the earth at a rate of 325 feet per week, laying concrete panels as they go.

The trains that run through those new tunnels will be equally state of the art. The vehicles, created by Siemens, will be automatic and driverless and can run up to 90 mph. Each is fully air conditioned, as are the stations, which are also all equipped with Wi-Fi. According to Saudi Arabia's High Commission for the Development of Arriyadh, roughly 20% of the system's power will come from solar energy harvested from cells at the stations.

Train to be used in the Riyadh Metro at Siemens plant in Vienna. Siemens The system's four main stations were each designed by different architecture firms from around the world. Zaha Hadid Architects is behind the Financial District station, which has six levels — four above ground and two below — and connects three different lines.

The rendering of the station's futuristic platform (below) puts older transit systems like New York City's subway to shame.