Aluminium plays a starring role in an ambitious project designed to connect the entire city, where plans for the $23 billion project include six color-coded train lines that will cross Riyadh with 177 kilometers of track served by 85 different stations.

What's unique about the project is that in most modern cities, the train system – whether in Paris, Tokyo or Chicago – was built and expanded over time, as their populations and urban environments changed. In Riyadh, home to six million people now with projections for a 50 percent increase by 2035, that is all happening at once so that an entire train and complementary bus system will launch in 2018 and help the country meet its sustainability goals.

In order to achieve that timeline, the Saudi government has awarded multiple concurrent contracts to architects, train car manufacturers, and construction project teams. The U.S. based Bechtel, a firm that helped build the English Channel Tunnel and the Athens Metro, was awarded the largest $10 billion contract.

The state-of-the-art train cars are built by more than one company too. Siemens is building 45 of them for Lines 1 and 2 as a part of the BACS consortium with Bechtel. Bombardier is making 47 of them for Line 3, and Alstom is manufacturing more than 60 cars for Lines 4, 5 and 6. The train cars for the driverless metro network have aluminium bodies – Bombardier confirms that theirs are made from recycled aluminium – with "jackets" in colors that match the train lines they run on.

They are also designed for the harsh Riyadh climate, with high-capacity air-conditioning systems, and seals and filters for traction systems, brakes, doors and other components that are specially designed to keep sand from getting in. That's usually not a consideration for systems anywhere else in the world!

As for the stations, contracts were awarded to multiple architects to complete the main train hubs with a unified architectural theme. Zaha Hadid Architects designed the King Abdullah Financial District station, for example, while Norway's Snøhetta has designed the Qasr Al Hokm Downtown Metro Station. Both architectural firms have created cultural centers that blend futuristic features and materials – steel, glass, aluminium and limestone – with sustainability commitments expressed in lush gardens and atria.

The Al-Olaya station is designed by Gerber Architekten of Germany, and captures the curves of sand dunes with a public rooftop garden that covers the entire roof of the structure as the trains arrive at the station in futuristic glass tubes. At a fourth station, designed by Omrania & Associates, the above-ground facility will host will host a bus station, market, mosque and public plaza, while continuing the lattice design elements and undulating surfaces that all stations, though unique, share in common.

Meanwhile, crews digging the underground tunnels are completing their work with giant tunnel boring machines. Last month, a ceremony was held at one of the major stations as another key stretch on the longest of the lines – the 40 kilometer Orange Line – was completed, bringing the entire project another 5.35 kilometers closer to its completion.