Sinking earth around a ground station caused the city's elevated light-rail line to suspend operations Thursday at three stations.

Officials from the Shanghai Metro Operation Company Ltd, which operates the so-called Pearl Line and two other metro routes, said the suspension on the concerned track portion and station closures will last at least two months.

The three affected stations -Shilong Road, Caoxi Road and Longcao Road - are at the southern end of the rail line.

The rail line has handled 230 million trips since it started service two years ago.

At present, the Pearl Line rail tracks are controlled by a system that mixes signals sent by people and computers, which makes it very hard for the company to increase the line's capacity.

Company officials say a new ATC (automatic train control) system has had been installed and is being tested.

Xu Jianqian, a professor at Shanghai-based Tongji University, which is well known for engineering studies, said the new ATC will enhance train speeds from 60 kilometres to 77 kilometres an hour. The time needed for travelling the distance between two stations will be shortened to 6 minutes from 7.5.

Xu said upgrading of the control system will improve efficiency and help ensure security of the line's operation and safety of passengers.

Another professor at Tongji University, who specializes in ground sinking studies, said ground subsidence and the track may be inevitable if Shanghai's soft land conditions aren't taken into consideration.

He added that sinking is not so serious that the safety of passengers will not be threatened.

"The grounds of some tall buildings in Shanghai also sink regularly, but their tenants do not have to be moved out," the professor said.

According to the professor, there are several possible reasons for the phenomenon, such as improper construction of high-rises close to the line and improperly built track foundations.