The Kalka–Shimla railway is a 2 ft 6 in (762 mm) narrow-gauge railway in North India which traverses a mostly-mountainous route from Kalka to Shimla. It is known for dramatic views of the hills and surrounding villages. The railway was built in 1898 to connect Shimla, the summer capital of India during the British Raj, with the rest of the Indian rail system. During its construction, 107 tunnels and 864 bridges were built along the route. The project’s chief engineer was H. S. Harington.

Its early locomotives were manufactured by Sharp, Stewart and Company. Larger locomotives were introduced, which were manufactured by the Hunslet Engine Company. Diesel and diesel-hydraulic locomotives began operation in 1955 and 1970, respectively.

The Kalka Shimla Railway, a 96.6 kilometer long, single track working rail link built in the mid-19th century to provide a service to the highland town of Shimla is emblematic of the technical and material efforts to disenclave mountain populations through the railway. The world’s highest multi-arc gallery bridge and the world’s longest tunnel (at the time of construction) of the KSR were the a testimony toof the brilliantce engineering skills applied to make this a dream a reality.

The railway has 864 bridges and a ruling gradient of 1 in 33, or three percent. It has 919 curves, and the sharpest is 48 degrees (a radius of 122.93 feet or 37.47 m). Climbing from 656 meters (2,152 ft), the line ends at an elevation of 2,076 meters (6,811 ft) in Shimla—a difference in height of 1,420 meters (4,660 ft). It originally used 42 lb/yd (20.8 kg/m) rail, which was later replaced with 60 lb/yd (29.8 kg/m) rail. One hundred seven tunnels were originally built, and 102 remain in use.

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