The Xinguanjiao Tunnel, the world's longest plateau rail tunnel on the Qinghai-Tibet Railway, was completed on Tuesday, local authorities said.

Spanning 32.645 km, the tunnel was finished on Tuesday afternoon, making it the longest rail tunnel in China, according to Zhi Changying, an official with the China Railway Tunnel Co. Ltd. (CRTC), a partner in the project.

The tunnel is part of the Qinghai-Tibet Railway's second trunk that links Golmud in northwestern Qinghai with Xining, the provincial capital. The 763.5-km-long line has been open to traffic since November 2010, except for the tunnel.

A project by the CRTC and the China Railway 16th Bureau Group Co., Ltd. (CRBGC), the tunnel took almost seven years to build. The "bottleneck" project posed challenges for engineers due to high altitude, complicated geological conditions and tight budgets, said Liu Hairong, an official with the CRBGC.

The tunnel, expected to open to traffic at the end of the year, will give a much-needed boost to the transportation capabilities of the high-profile Qinghai-Tibet Railway, which connects the northwestern Qinghai Province and Lhasa, capital of southwest China's Tibet Autonomous Region, Zhi Changying said.

"It will allow the railway to better connect Qinghai and Tibet," Zhi said, adding that the tunnel is another improvement in the railway network in western China.