The second south-to-north subway line in China's capital started trial operation on Monday, according to municipal transport authorities.

An opening ceremony was held Monday morning for the No. 4 metro line in Beijing, which traverses 28.2 kilometers from the Gongyixi Bridge at the southern section of the city's fourth ring-road to the Summer Palace and the Old Summer Palace.

The project cost 15.38 billion yuan (2.25 billion U.S. dollars) to build, with Beijing Government taking 70 percent of the stake and the MTR Corporation from Hong Kong holding the remaining 30 percent.

This was the first time private capital was ushered in for construction of a subway line on the Chinese mainland.

It takes 48 minutes to run through all the 24 stations on the No. 4 metro line, and the distance is usually a two-hour drive for a car.

It is estimated that the new subway line will accommodate 400,000 passengers a day on average.

According to the local transport authorities, No. 4 subway is the first metro line on the mainland to adopt the state-of-the-art shock-absorption and noise-reduction technology. The technology, which enables a subway train running "quietly", will be introduced to construction of new metro lines in Beijing, the authorities said.

Li Xiaosong, spokesperson for the municipal transport authorities, said from now to 2015, Beijing will spend 270 billion yuan to improve public transport infrastructures, including subway lines.