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Siemens says Saudi train order worth 1.5 bln euros

* Siemens to supply driverless trains, related systems

* Subway to have strong air conditioning, sand-proof seals

* Order is part of 7.5 bln euro contract already announced

* Total of $22.5 bln worth of orders awarded in July

FRANKFURT, Oct 10 (Reuters) - Siemens's share of a major contract to supply trains for a new subway system in the Saudi Arabian capital of Riyadh is worth 1.5 billion euros ($2 billion), the German group said on Thursday, confirming its biggest ever rail engineering order from the region.

The contract is part of a total of $22.5 billion worth of orders awarded to three foreign-led consortia by Saudi Arabia in July for the design and construction of the metro rail system in Riyadh.

The project, which will involve six rail lines extending 176 kilometres (110 miles) and carrying electric, driverless trains, is the world's largest public transport system currently under development, Saudi officials have said.

Siemens said in a statement on Thursday it would supply driverless subway trains, electrification systems and signalling technology for two of the six lines.

Flush with cash after more than two years of high oil prices, Saudi Arabia is pumping billions of dollars into infrastructure projects designed to improve living standards and ease social discontent in the wake of the 2011 uprisings elsewhere in the Arab world.

The government also says it wants to upgrade the country's infrastructure to help the economy diversify beyond oil, making it less vulnerable to any future plunge of global oil prices.

The German engineering group will deliver 74 subway trains equipped with an extra-strong air conditioning system to cope with the extreme heat as well as special seals and filters in brakes and doors to cut down on the amount of sand that makes its way into the train cars.

The order is part of a 7.5 billion euros ($10.14 billion)contract awarded to a consortium led by U.S.-based Bechtel.

Other major orders for the subway system included a $7.82 billion contract awarded to a consortium led by Spain's Fomento de Construcciones y Contratas and a $5.21 billion order for a group headed by Italy's Ansaldo STS.