Construction work has begun on a railway link passing through the foothills of Kanchanjungha mountain range and Tiesta river valley in India.
The project, estimated to cost Rs3300 crore ($727m), is being executed by New Delhi-based Ircon International.
More than 70% of the 44.4km track, connecting Rangpo and Sivok, will be through tunnels.
Bridges crossing deep gorges and valleys will also be required with bridge piers of over 70m.
The railway link is expected to improve connectivity to the northern Indian state of Sikkim.
The work is scheduled for completion in five years.