The Railway line from Mettupalayam to Ooty is 46 km. long and lies partly in Coimbatore District and partly in Nilgiri District of Tamilnadu, on the eastern slopes of the Western Ghats. Mettupalayam is located at the foothills with an elevation of about 330 metres and Udagamandalam (Ooty) on the plateau with an elevation of 2200 metres. The steepest gradient on this line is 1 in 23 and there is no rack on it as provided between Coonoor and Mettupalayam. The average gradient of this line is about 1 in 24.5. The gauge is laid to Metre gauge (1000 mm). The sharpest curve on the section is 17.5 degrees. The track is made of 50 lb rails. There are 250 bridges on the section, out of which 32 are major ones and 15 are road over bridges. There are 16 tunnels between Kallar and Ooty all of which are in excellent condition. The section from Mettupalaiyam to Udagamandalam is provided with stone ballast and the cushion varies from 75 mm to 150 mm.

The Maximum permissible speed on Mettupalaiyam-Kallar and Coonoor-Udagamandalam "Non-Rack" system is 30 while between Kallar and Coonoor, the maximum permissible speed is 13 kmph. This rail system is one of the steepest in Asia. To overcome this incline it uses a unique ABT (Alternate Biting System) commonly known as `rack and pinion' rail system. In this system, between the two conventional rails, there are two rack bars, that step with each other, on which the pinions of the locos and coaches 'climb' in a similar manner as a person climb a ladder using his feet alternately. In this train, the steam engines were used. The steam engine pushes the train from behind while ascending and is in front that stop it from running amuck when going downhill. Presently all traffic on the rack section is in the able hands of eight X class locomotives built by SLM of Switzerland. These are compound locomotives and have four cylinders. The main cylinders use high-pressure steam and drive the wheels of the loco whereas the smaller cylinders using low-pressure steam drive the rack system. The youngest X class loco is over 50 years old and the oldest is 80 years old against the average of 25 years working life of a steam loco.

History of the Nilgiri Mountain Railway
In 1854, the first plans were made to build a mountain railway from Mettupalayam to the Nilgiri Hills. But it took about 45 years to complete the construction and installation of the line. The line from Mettupalayam to Coonoor was completed and opened for public in June 1899. It was operated first by the Madras Railway under an agreement with the Government. The Madras Railway Company continued to manage this railway line on behalf of the government for a long time until the South Indian Railway company purchased it. The line was extended to Ootacamund from Coonoor in 1908 on the same gauge over a distance of 11 and 3/4 miles at a cost of Rs. 24,40,000. These expenses were heavy as in addition to the tunnels, a big bridge over the river Bhawani at the foothills was need to be built. Besides this large bridge, 26 other bridges smaller in size, were constructed and heavy expenditure was incurred in rock-cutting and blasting.

Earlier, the train journey from Chennai to Metupalayam took just over 17 hours and cost Rs 20 first class and another 20 rupees to cover the remaining 33 miles up the steep mountain road to Coonoor and Ootacamund by the ‘Nilgiri carrying Company’s Mail and Express Tonga Service’. The
heavy baggage had to be sent by the bullock cart and the only alternative at that time was to hire a pony and arrange for luggage to be taken up by individual baggage carriers using the shorter but even steeper old road to Coonoor. Nowadays, the traveller from Ootacamund leaves Chennai central station on the evening by the Nilgiri Express and arrives at Mettupalaiyam in the morning. From here, he just travel through the Nilgiri Mountain railway and reaches Udagamandalam in less than 15 hours.