STANDING 23 metres above the ground, Anantha Guru, the site inspector at an under-construction viaduct of the Mumbai-Ahmedabad High Speed Corridor in Vaktana village near Surat, discusses a “mystery” with a group of engineers. The material that lies sandwiched between the track bed and the track slabs that hold the rails has a “secret ingredient” that allows trains to run at blinding speeds of 320 km per hour. Only the Japanese know what it is, they say.

Guru walks towards the Cement Asphalt Mortar (CAM) injection car, a machine brought from Japan that’s being used for the first time on Indian soil. Its task is precise. Before the rails can be laid on the track slab, it creates a 40-100 mm cushioning layer of mortar that absorbs the vibrations generated when trains run at such high speeds.

“With this machine, we inject CAM, a mixture of nine ingredients such as cement, asphalt, polymer emulsion, sand, water, and other materials. Three of the ingredients in CAM — cement, sand and water – are available in India; the rest comes from Japan,” Guru explains.

That’s the missing link. “They (Japanese) mix something into the asphalt emulsion. They have not told us what it is,” says another engineer, adding that researchers at IIT Kharagpur are now studying the material to decode it.

Almost a decade after Prime Minister Narendra Modi laid the foundation stone for the bullet train project in September 2017, the country’s high-speed dreams are finally taking shape in the form of the 508-km Mumbai-Ahmedabad corridor.

The Rs 1.98-lakh-crore Mumbai-Ahmedabad line is the first of eight high-speed rail projects. The government hopes to build seven other lines, spanning 4,000 km, at a cost of Rs 16 lakh crore. The Mumbai-Ahmedabad project has Japan as the primary partner, with loans from the Japan International Cooperation Agency (JICA) and Japanese technical experts training Indian engineers. As part of the initial deal signed in 2017, Japan was to supply its famed Shinkansens. However, amid growing “uncertainties”, sources said, India has already started building its own bullet trains at a BEML plant in Bengaluru.

After multiple delays, the Mumbai-Ahmedabad High Speed Rail corridor is expected to be operational by 2029. The first of the stretches, the 48 km from Surat to Bilimora, is scheduled to be completed by August 2027.

Overlooking the quiet flow of the Sabarmati is a 480-metre-long viaduct (bridges on which elevated railway lines or roads run). Over 90%, or 465.38 km, of the high-speed network’s 508-km-long tracks will run on viaducts such as these, at heights of over 20 metres from the ground, making a journey in the bullet train a ride through the skies.

A senior officer of the National High Speed Rail Corporation Limited (NHSRCL), the government organisation responsible for building and managing the high-speed rail corridors, said that unlike in European countries, the bullet train’s tracks had to be elevated for safety reasons. “We could not have run the bullet train on the surface. People and animals cross the railway line. Even on the railway’s broad gauge lines, where trains can go up to 160 kmph, barricades are installed on either side of the tracks. Here, in the case of the bullet train, we are talking of speeds of 300 kmph and upwards,” the officer said.

The under-construction viaduct at Sabarmati river, which leads to one of the terminal points of the railway line, overlooks three other bridges. The closest is an old railway bridge, its tightly spaced piers (the vertical columns or pillars that support a viaduct) reminiscent of an earlier era of rail construction.

Vijay Behera, Deputy Chief Project Manager, at the site, said that before the seven piers of the viaduct were built, engineers had to build piles (deep foundations to support heavy bridge loads) underwater. “For every pier, we have to cast 16 piles under the Sabarmati. This is one of the most difficult projects I have ever worked on. The bullet train itself will be light, but since it will have to travel at such high speeds, other dynamic forces such as air resistance, vibrations, and centrifugal forces will come into play. So the bridge has to be really strong,” he says.

A few kilometers from the Sabarmati viaduct is a steel bridge that’s being built over a road bridge. The structure, 80 metres long, is one of 28 steel bridges planned along the corridor. The bridge, weighing 1,004 metric tonnes, has seismic stoppers installed at its piers to help it withstand earthquakes. More than 36,000 bolts hold the structure together.

While Japan moved directly from metre gauge (a narrow gauge in Japanese standard) to high-speed networks, India took the longer route, moving from narrow and metre gauge to broad gauge (maximum speeds of 160 kmph) that now form over 97% of the rail network. The next step, high-speed, took years to take off.

At Ahmedabad, a slice of that journey is visible at an elevated level. Two lines of the Ahmedabad railway station, which were under conversion from metre gauge to broad, were taken over by NHSRCL. The Ahmedabad station for the high-speed rail is coming up here, above the older lines.

At a 14-acre factory in Kim, Surat, Arjun Yadav, a site worker from Uttar Pradesh, fixes his digital depth caliper to one side of a 4.5-tonne track slab to measure its thickness. The track slab is the bed on which the high-speed rails will rest. The machine reads 191 mm. Perfect, so he marks “OK” on its surface.

Even a minor variation would have made it redundant. The factory was set up in September 2023 to manufacture track slabs, one of two such facilities in Gujarat, for the Vapi-Vadodara stretch of the bullet train. At peak capacity, the facility can cast 120 track slabs a day.

The slab Yadav measured is the 36,112th track slab manufactured at the Kim factory. The Vapi-Vadodara stretch will need 96,000 track slabs, of which 36,000 have been supplied so far.

Ravindra Singhal, a senior track expert at Kim, says three types of machinery have been deployed at the site — Japanese, Italian and Indian. “In Japan, a factory of this nature casts 20-30 slabs a day. Ours is much larger. The Japanese have a factory every 20-30 km. For this project, we have two factories so far. Around 1,000 people have been trained for this work by JARTS (Japan Railway Technical Service),” says Singhal.

Sanchit Garg, overall incharge and joint general manager of IRCON, said 16 engineers had arrived from Japan to train engineers at the Kim facility.

It’s a partnership that’s evident at the HSR Depot in Surat-Niyol, nearly 30 km away. The depot is where the high-speed coaches will come for routine inspections once the project takes off.

Two engineers from Japan, Hokutu and Nake Mura, are at the facility to train Indian engineers to operate a wheel lathe machine.

Hokutu, who has previously worked on Joetsu Shinkansen, later explains that the bullet train’s steel wheels wear out from repeated contact with the tracks at high speeds. The wheel lathe machine restores the wheel’s shape by shaving off a very thin outer layer of metal, a process called reprofiling. “A new wheel starts with a diameter of 860 mm. Every reprofiling makes it smaller, but once it reaches 790 mm, the wheel can no longer be used,” he says.

“The machine was originally designed by the Germans, then the Japanese learned from them. Now we are passing the knowledge on to India,” he says.

In the making of the bullet train are technological advances of the kind India has rarely witnessed before. Over a century after the British laid railway lines across India, using hard labour to lay thousands of kilometres of tracks, these high-speed lines are being put together by machines that tower over the landscape and the workers, steered by switches and engineers.

As the project enters Maharashtra, the landscape changes. In Basantwadi village, an Adivasi belt near Dahanu in Palghar district, the high-speed line cuts through the foothills of the Sahyadris. Tunnels replace viaducts and engineering becomes more complex.

Around 26.22 km (5.16%) of the 508-km high-speed line runs through tunnels. Of this, 21 km is in Mumbai and includes India’s first 7-km undersea tunnel that runs beneath the Thane creek.

Engineers of L&T working at Tunnel No. 6, a 380-metre-long mountain tunnel at Basantwadi, say the geological composition of the area – young basalt rock, mixed with layers of soil – makes excavation difficult and unpredictable. Work on the tunnel started in January 2025 and breakthrough was achieved in 350 days.

“The maximum overburden — the thickness of rock above the tunnel — is just 20 meters. For nearly 250 meters of the stretch, it drops to less than 14 metres, making the excavation very difficult. For the first time, we have an automatic, real-time monitoring system that tells us in advance if there is any possibility of tunnel collapse,” he says.

Deen Dayal Mathuria, project director of L&T at the site, said the tunnel, with a horseshoe shaped opening, will have a hood at the entrance. “At high speeds, a train entering a tunnel creates a powerful pressure wave and generates a loud, explosive sound. The entrance hood helps release this pressure gradually and reduce the noise,” he says.

Nearly 20 kilometers from the tunnel, in Gowana village, Palghar district, a massive launching gantry is at work, picking 980-tonne girders, each 40 meters long, and placing them into position. One after another, these segments come together to form the viaduct or bridge, the spine on which the train will run.

At the centre of this operation is the girder transporter — a 400-tonne, 216-wheel carrier that has been designed and manufactured by L&T for the project. It moves slowly, much like the Hunter Killer machines in the Terminator, but with far gentler intent, as it carries girders from the casting yard, 2 km away, and shifts it to the launching gantry. The launching of a single girder takes about an hour.

From his house in Gowana, a village of around 6,000, Sachin Pardi can see the blue arms of the launching gantry. “Mar jaayenge, lekin kabhi iss train mein nahi baith paayenge (We will die, but may never get to ride this train). I have to be born again to sit in the train,” smiles Pardi, a paddy farmer.

Work on the viaduct continues through the night. With it, the constant hum of machinery and a siren that goes out loud intermittently before every new activity. “There is a lot of noise. It is very difficult for us to sleep,” he says.

At Shilphata in Mumbai, the tunnel passes beneath critical urban infrastructure – two major water pipelines and a 610-mm gas pipeline.

The tunnel cuts through the Parsik hills. In some sections, the thickness of the rocks above the tunnel was 160 metres, but elsewhere, it would drop to as little as 20 metres. At one particularly sensitive point, officials said, only 4 metres separated the tunnel from the pipelines above.

“An error in excavating the tunnel could have damaged these pipelines. Here, the excavation had to be carried out manually,” says Krishan Singh, Deputy Chief Project Manager at the site.

Inside the tunnel, an Ethylene Vinyl Acetate membrane brought in from South Korea has been installed as waterproofing. Earlier tunnels relied on PVC membranes.

“For this 21-km tunnel, we mapped over 600 buildings overground to document existing cracks. Before carrying out the blasting, instruments such as crackmeters, tiltmeters, and seismometers were installed across nearly 50 buildings. At most, there was a slight vibration during the blasting process, like what you feel standing on a bridge,” said a senior NHSRCL engineer.

The project ends at Bandra Kurla Complex (BKC), the only underground station of the project. The worksite resembles a gigantic cavern, like a part of the earth has been gouged out. Outside this cavern, the multi-storeyed buildings of the business district seem diminished, resembling a child’s LEGO set.

At Mumbai, India’s railway story comes full circle. It was in Mumbai that the country’s first passenger train ran between Bori Bunder (present-day Chhatrapati Shivaji Maharaj Terminus) and Tannah (now Thane) in 1853. The city will now host the terminus of India’s first high-speed train.