Travellers on the Mumbai-Ahmedabad Bullet Train may never notice them directly, but a new piece of engineering being introduced on the corridor could make high-speed journeys quieter and more comfortable.

As per the latest update from the National High Speed Rail Corporation Limited (NHSRCL), they are installing tunnel hoods at mountain tunnels along the Mumbai-Ahmedabad Bullet Train route. This is the first time this technology has been used in railway tunnels in India. The bullet train corridor passes through seven tunnels in Maharashtra and one in Gujarat, with tunnel hoods being installed at both ends of these tunnels.

This technology, being utilised in Indian Railways tunnels for the first time, is expected to decrease noise around tunnel portals and enhance passenger comfort. It will also help trains to operate more efficiently at speeds exceeding 300 kmph.

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What are tunnel heads, and why are they required?

A tunnel hood is a specially designed structure constructed at the entrance and exit of a tunnel. It plays the role of a transition zone between the open environment and the enclosed tunnel space.

According to the NHRSCL official statement, when a high-speed train enters a tunnel, it pushes a large volume of air ahead of it, much like a piston moving inside a cylinder. This sudden compression creates pressure waves that travel through tunnel.

When these pressure waves come out at the other ned, they can cause a loud booming sound, often termed as “tunnel boom”. Tunnel hoods are designed in a way to reduce this effect by allowing pressure to change more moderately.

NHSRCL said tunnel hoods help in controlling the pressure changes generated by high-speed trains and enhance the overall aerodynamic performance of the system.

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In simple terms, they help the train and the nearby air adjust to each other smoothly instead of abruptly. Thus, this makes the high-speed rail operations quieter and more efficient.

How do tunnel hoods function?

The tunnel hoods being installed on the bullet train corridor consist of specially designed pressure-relief openings or windows.

When a train enters a tunnel, micro-pressure waves travel through the enclosed space. The openings in the tunnel hood allow part of the compressed air to escape slowly into the atmosphere rather than building up inside the tunnel.

As displayed by NHRSCL, in a phase-wise manner, at first, the train enters the tunnel, pressure waves travel through the tunnel and the train exits through the tunnel hood. By releasing air gradually, the structure reduces the intensity of pressure waves and helps prevent the booming effect at the tunnel exits.

The tunnel hoods constructed for the project vary in size depending on the tunnel. NHSRCL stated that the structures will extend roughly 30-45 metres beyond tunnel portals and include multiple pressure-relief openings. The longest mountain tunnel section on the route will consist of the largest tunnel hoods and the highest number of pressure relief windows.

How big are the tunnel hoods?

As per NHSRCL, tunnel hoods on the Mumbai-Ahmedabad Bullet train corridor will extend 30-45 metres beyond tunnel portals on each side. It will comprise 20-26 pressure-relief windows at each end.

The MT-8 tunnel in Valsad will consist of 31-metre-long hoods on each side with 20 windows at both ends. The mountain tunnels in the Palghar district will have hood lengths varying from 30 metres to 45 metres and 20-26 pressure relief windows. The largest tunnel hood installation will be at 2.66 km long MT-4 and MT-3 tunnel section in Palghar. In this portion, the hoods will extend 45 metres on each side and include 26 windows at each end.

Is this technology used elsewhere?

Tunnel hoods are commonly used on high-speed rail networks in countries operating bullet trains, where trains regularly travels at speed above 30 kmph.

Their adoption on the Mumbai-Ahmedabad Bullet Train project reflects the use of advanced engineering solutions and international best practices aimed at improving safety, passenger comfort, and environmental performance.

The Mumbai-Ahmedabad corridor, India’s first bullet train project, is being developed using Japanese Shinkansen technology.