CHONGQING, Dec. 26 (Xinhua) -- A high-speed railway line was put into operation Thursday, connecting southwest China's Chongqing Municipality and a city in the central province of Hunan.

The 335-km-long rail line links Qianjiang district of Chongqing and the city of Changde in Hunan, passing seven railway stations with a maximum speed of 200 kph in its initial phase of operation.

The railway is part of a major high-speed rail line connecting Chongqing; Changsha, capital of Hunan; and Xiamen in southeast China's Fujian Province.

The design of the railway started in 2009 and its construction began in December 2014.

"We have successfully solved the global problem of how to pass rail lines through treacherous karst regions," said Liu Tingxi, chief designer of the rail line, referring to the limestone region with sinkholes, underground streams and caverns.

About 143 km of the rail line passes through the karst region, said Liu, adding that "it is where the attractive scenery is, but also where the risks for the project lie."

Due to the mountainous terrain of the region, constructors built 104 tunnels and 196 bridges, whose total length account for about three-quarters of the rail line.