[1] The basis of UNESCO's designation of the Mountain Railways of India as a World Heritage Site is "outstanding examples of bold, ingenious engineering solutions for the problem of establishing an effective rail link through rugged, mountainous terrain.

The three routes together have been titled the Mountain Railways of India under UNESCO World Heritage Site criteria ii and iv, within the Asia-Pacific region.

The Matheran Hill Railway, a fourth mountain line, has been nominated and is pending approval by the international body.

The latter is a major summer hill station and the centre of a flourishing tea-growing district located in West Bengal.

On the recommendations of a committee appointed by Sir Ashley Eden, work on the route began in 1879 and was completed by July 1881.

[5][7] The line underwent several improvements such as making its gradients more gradual over the years to increase its manoeuvrability.

By 1909–1910, the Darjeeling Himalayan Railway was carrying roughly 174,000 passengers and 47,000 tons of goods annually.

[6] Important features incorporated in the line include four loops (spirals) and four 'Z' reverses (zigzags).

In 1897, a major earthquake damaged the railway, requiring rebuilding of the route, including extensive improvements to the track and stations.

The route is located within the state of Tamil Nadu and travels through the Nilgiri Hills, which are popularly known as the Blue Mountains of Southern India.

[9][10][11] The Nilgiri Mountain Railway was declared a UNESCO World Heritage Site in July 2005.

Initially, the town of Coonoor was the final station on the line, but in September 1908 it was extended to Fernhill followed by Udagmandalam by October 15, 1908.

[12]A unique feature of the line, which is still fully operational, is that its oldest and steepest track uses rack and pinion technology.

Currently, the line runs for 7.2 km (4.5 mi), up to the foothill station of Kallar, where the rack rail portion begins.

[13] Shimla is the modern capital of Himachal Pradesh[9][14] and is at an elevation of 2,205 m (7,234 ft) in the foothills of the Himalayas.

Until the 1980s, the railway was closed during the monsoon season (because of the increased risk of landslides) but is now open all year.

The Kangra Valley Railway is 2 ft 6 in (762 mm) narrow-gauge railway and covers a distance of 163 km (101 mi) between Pathankot and Joginder Nagar, an area known for its nature and ancient Hindu shrines, in the sub-Himalayan region.

The route crosses major earthquake zones and is subjected to extreme temperatures of cold and heat, as well as inhospitable terrain, making it a challenging engineering project.