Indian engineers are preparing to slide a bridge deck across a remote Himalayan valley aiming to complete the world's highest arch railroad crossing in 2023. Difficult terrain, harsh weather and the threat of terrorism extended construction of the Chenab River bridge to nearly two decades.

So inaccessible was the valley in India—administered Jammu and Kashmir—that engineers preparing to bid for the contract had to travel on horseback. In 2004, construction was expected to last about three years, but the project reached its peak only this April when the last steelwork section completed the arch. Prime minister Narendra Modi hailed the event as marking India’s growing engineering prowess.