BEIJING - China's railway freight volume, a key indicator of economic activity, dropped in the first three quarters of 2015, the country's top economic planner said on Monday.

The railways transported 2.53 billion tons of cargo from January to September, down 11.4 percent year on year, compared with a decrease of 10.9 percent in the first eight months, and a 10.1 percent drop in the first half, according to the National Development and Reform Commission.

In September alone, railways transported 270 million tons of cargo, down 15.6 percent year on year.

The figures came as China's factory activity fell into contraction territory in September, the fourth time this year.

China's official purchasing managers' index (PMI) came in at 49.8 in September.

A reading above 50 indicates expansion, while that below 50 represents contraction.

The country's economy posted a 6.9-percent growth year on year in the third quarter of 2015, lower than 7 percent in the first half of the year, which is the first time the quarterly growth rate had dropped under 7 percent since the second quarter of 2009.