The third line in Dubai's metro system, which is due to open in September 2009, will present new technical and financial challenges for all those seeking to capitalise on the great Arabian railway boom.

As Abdul Redha Abu Al Hassan of Dubai's Roads & Transport Authority (RTA) confirmed at MEED's rail projects conference last week, the 52-kilometre Purple Line between Dubai International Airport and Al Maktoum Airport near Jebel Ali will be entirely underground.

This will entail boring either two or four tunnels to carry a twin-track commuter line and a separate twin-track high-speed link between the two airports. The advantages are obvious: the disruption caused in constructing surface railway lines and the problems they present for town planners when finished will be avoided. The problems are equally obvious: never before has long-distance tunneling been attempted in Arabia, and every kilometre will cost up to three times more than surface railways.

But Dubai has few options. The population of the city, now about two million people, could rise to 10 million in not much more than a generation if all announced plans forge ahead. The city is already struggling with the world's highest rate of car ownership. Unless mass public transport comes, the new Dubai will not work.

The purple line signals that Dubai's urban rail plans are entering a new era. The red and green lines can now be seen as representing no more than an initial first attempt to keep the city moving. The scale and diversity of Dubai's transport needs are so enormous that a comprehensive approach is required.

According to MEED's investigations, more than half a dozen of Dubai's real estate developers plan to build rail systems of some kind. These will have to be integrated into a city-wide network that is affordable and efficient. Consequently, the RTA is about to commission a comprehensive railway study to help define a long-term strategy.

It is just one part of the world's greatest railway development programme. According to MEED, more than $100bn worth of railway projects are under way or planned in the GCC. For Arabia, this is the age of the train, at last.

The global railway industry, convinced that many of the projects will go ahead, is now making tracks to the region. In the next two years, they will see the first three begin operating:

• The Palm Jumeirah monorail in the first half of 2009

• Dubai Metro's red line in September 2009; the green line in March 2010

• Saudi Arabia's north-south railway (NSR), from close to the border with Jordan to Riyadh and Ras al-Zour on the Gulf, in October 2010.

More details of other major projects will soon be unveiled. A decree is expected within months to set up Union Rail, the company that will be empowered to build a 325-kilometre railway line connecting all seven emirates of the UAE. It will run from Fujairah to Abu Dhabi's border with Saudi Arabia.

And on 15 October, GCC transport ministers approved a feasibility study for the GCC railway line that will follow the Arabian littoral of the Gulf coast from Kuwait to Oman. Hopes are high that the plan will be approved by the next GCC summit.

The new frontier for Gulf railways is urban railway schemes. The MEED railway conference was told that a surface transport masterplan for Abu Dhabi, including Abu Dhabi city, will be completed in February.

Initial proposals call for a circular metro line that will connect Abu Dhabi international airport with the central business district via Yas, Saadiyat and Reem islands, the exhibition centre and Masdar City. There may also be an underground light rail system in the central business district.

Rail systems are planned by some of the large Abu Dhabi developers. Tests of Abu Dhabi's Masdar City personal rapid transport (PRT) system are due next September. When it is complete, the city will have the most complex PRT system in the world with 3,000 vehicles.

Rail systems are planned in Doha and Kuwait and there have been preliminary discussions about a monorail in Bahrain. Saudi Arabia has the largest number of projects. In addition to the NSR, there is the Saudi Landbridge between Jeddah and the Gulf and the Mecca-Medina railway. Light rail projects have been proposed for Riyadh, Jeddah, Mecca, Medina, Hail and the King Abdullah Economic City in Rabigh.

When these are complete, further challenges will emerge, notably how the railways will be operated and financed long-term. But it seems a page has been turned in Gulf transport history. A century after work on the Hejaz line between Damascus and Medina was completed, Arabia has finally decided that railways are the right way to go.