The 3,345-meter-long tunnel, located about 80 kilometers from Lhasa, capital of southwest China's Tibet Autonomous Region, was built at a place at an altitude of 4,264 meters above the sea level.

The Work of construction on the project started on November 1, 2001, and its longest tunnel was completed last Wednesday. The whole tunnel linked up last October, and workers then dug ditches and cable troughs in the tunnel.

It is a hard job to build the tunnel as it is situated on the " roof of the world" with complex geological structures, plenty of water and frigid climate, noted Wang Shunping, an official in charge of the railway construction.

To tackle these knotty problems, workers have added water-proof planks, installed ventilation fans and erect oxygen machines at the entrance of the tunnel to protect workers' health.

The Qinghai-Tibet railway, the highest rail route on earth, runs from the city of Golmud in western Qinghai province to Lhasa in Tibet with a total length of 1,142 km. About half of the railway tracks were laid on permanent frozen earth. The project is scheduled to be completed in six years at a total investment of 26. 2 billion yuan (about 3.1 billion US dollars).

The track-laying project will commence in Tibet this month, enabling the region to end the history of no railways.