Building a railway from the plains of West Bengal to Darjeeling presented enormous engineering challenges.
To gain elevation without making the gradient excessively steep, railway engineers incorporated unusual track arrangements.
UNESCO records six zigzag reverses and three loops along the Darjeeling Himalayan Railway.
At a zigzag, the train changes direction before continuing its climb at another level. Loops allow the railway to curve around and gain height within a relatively limited area.
These clever nineteenth-century engineering solutions helped make railway transportation through the difficult Himalayan landscape possible