Workers building a tunnel for the world's longest heavy freight railway, which will stretch from coal-rich northern China to the country's east coast, achieved breakthrough five months ahead of schedule on Sunday morning, according to the project contractor.

The Dazhongshan Tunnel runs for 14.5 kilometers and is being built in Sanmenxia, Henan province, by Shanghai Civil Engineering Co, a subsidiary of China Railway Engineering Corp.

It is an essential part of China's vast coal freight rail project, which is expected to carry about 200 million metric tons a year from the Inner Mongolia autonomous region to Jiangxi province.

Construction of the 1,837-km railway began in 2015, with total investment expected to reach about 190 billion yuan ($29.7 billion). It will run through seven provinces and regions, stopping at 80 stations along the route, the contractor said. The work is scheduled to be completed in October 2019.

Zhang Lin, chief engineer for the Dazhongshan Tunnel project, said the rail project is a massive challenge as the route crosses the Yellow River (twice), the Yangtze River and complex mountain ranges.

Dazhongshan Tunnel is in Sanmenxia's mountainous Lushi county. "Most of it is buried 80 to 500 meters from the surface, and the deepest point is 710 meters," Zhang said. "Digging the tunnel has been hard and risky."