How India’s Bullet Train Is Being Built: Full Span Girders, J-Slab Track and the Undersea Tunnel

India’s first bullet train is no longer a drawing. By September 2026, the Railway Minister reported 371 km of viaduct and 456 km of piers on the 508 km Mumbai–Ahmedabad corridor. Most news stories stop at the numbers. This guide explains how the corridor is actually being built: why almost all of it is elevated, how a 970 tonne girder is placed in one lift, what “J-slab” track is, and how the 21 km tunnel under Mumbai and Thane Creek is being dug. Every figure is dated, because they change every month.

The corridor in one paragraph

The Mumbai–Ahmedabad High Speed Rail (MAHSR) is built by the National High Speed Rail Corporation Limited (NHSRCL) using Japanese Shinkansen technology. It runs 508 km from an underground terminal at Bandra Kurla Complex (BKC) in Mumbai to Sabarmati in Ahmedabad, with 12 stations. The Railway Minister has said the maximum speed will be 350 km/h with trains operating at 320 km/h, and the full journey is expected to take about 1 hour 58 minutes. Of the 12 stations, BKC is underground and the other 11 are elevated.

Why the line is almost all viaduct

About 465 km of the 508 km is on viaduct. An elevated line looks expensive, but on this route it solves several problems at once:

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• Land: a viaduct needs a narrow strip for pier foundations, not a wide embankment. On a densely farmed and built-up coastal belt, that matters.
• No level crossings: a 320 km/h train cannot share the ground with roads, cattle and people. Going over everything removes the conflict.
• Soft soil and flooding: much of the route crosses soft coastal ground and seasonal rivers. Deep pile foundations and a raised deck avoid the long-term settlement and flood risk of a high embankment, and settlement is the enemy of a track that must stay within millimetres.
• Repetition: hundreds of kilometres of identical 40 m spans can be made like a factory product. That is what makes the full span method possible.

Full span launching: a 970 tonne girder in one lift

Most metro viaducts in India are built segment by segment: short precast pieces are hung from a launching girder and stressed together. MAHSR uses that too in some places, but the bulk of the viaduct uses full span launching. Each span is a single 40 m prestressed concrete (PSC) box girder, cast in one piece in a casting yard near the alignment. NHSRCL figures put each girder at about 390 m³ of concrete and 42 tonnes of steel, for a weight of roughly 970 tonnes.

The sequence works like this:

1. Cast and cure in the yard. The full girder is cast in a reusable steel mould, cured and prestressed under controlled conditions. Quality is far easier to control in a yard than on top of a pier.
2. Lift and stack. A straddle carrier, a heavy gantry crane running on rails in the yard, lifts the finished girder and moves it to storage or to the loading point.
3. Transport along the finished deck. A girder transporter, a multi-axle vehicle, carries the girder along the spans that are already in place to the working face.
4. Launch. A full span launching gantry standing on the piers takes the girder from the transporter, moves it forward and lowers it onto its bearings. The gantry then moves itself forward to the next span. The first few spans out of each yard are placed with a pair of bridge gantries before the transporter can run on the deck.

The Railway Minister has called the method about ten times faster than conventional segmental construction, and the cranes that lift more than 1,000 tonnes are designed in India. The trade-off is that the method needs very large casting yards, very heavy equipment, and a run of standard spans. Where the line crosses rivers, highways and railways, or needs longer spans, other methods take over.

Where full span does not fit

• Segmental spans: precast segments joined on site are used where spans are not standard or access for full span equipment is difficult.
• Steel bridges: over existing railway lines and highways, steel truss bridges are assembled and pushed into place so traffic below can keep running. NHSRCL launched its first steel bridge in Maharashtra in September 2026.
• River bridges: large rivers such as the Narmada, Mahi and Sabarmati need well or pile foundations in the riverbed and longer spans. The Sabarmati bridge was built by the balanced cantilever method, which we explain in our Sabarmati river bridge article. In July 2026 the Ministry reported 21 of 25 well foundations on the Narmada bridge and 11 of 12 on the Mahi bridge.

J-slab track: why there is no ballast

Ordinary Indian Railways track sits on crushed stone ballast, which needs regular tamping to keep its shape. At 320 km/h, the track geometry has to stay almost perfect, and ballast can also be thrown up by fast trains. MAHSR therefore uses Japan’s J-slab ballastless track, with the concrete slabs made in Indian casting yards.

In the Japanese slab track system, the layers from bottom to top are typically:

• Reinforced concrete track bed, cast on the viaduct deck.
• Cement asphalt mortar (CAM) layer, injected under each slab. It fills the gap, adjusts the final level and adds some resilience.
• Precast track slabs, positioned by stoppers so they cannot slide along or across the track.
• Fastenings and rails on top, with the fastenings allowing small final adjustments.

The benefit is low maintenance and stable geometry for decades. The cost is that the structure underneath must not settle, because you cannot simply tamp a concrete slab back into line. That is one more reason for the deep foundations and the elevated alignment. As of 22 July 2026, the Ministry reported 209 km of track bed laid, and about 87% of the track bed on the Surat–Vapi stretch.

The 21 km tunnel and the undersea section

In Maharashtra, the line leaves BKC in a 21 km tunnel, of which about 7 km runs beneath Thane Creek. Two tunnelling methods are used:

• Tunnel boring machines (TBMs) for the long bored sections, including under the creek. Two TBMs were launched in July 2026 from Vikhroli and Sawli. Each weighs more than 3,000 tonnes, is 96 m long and has a 13.6 m diameter cutterhead.
• Mountain tunnelling, also known as the New Austrian Tunnelling Method (NATM), on a section between Ghansoli and Shilphata. NATM excavates in stages and supports the rock with shotcrete, rock bolts and steel ribs, letting the ground carry part of its own load. About 5 km had been excavated this way by September 2026.

The BKC station itself is about 32 m deep, with three levels and six platforms, each more than 400 m long. For more on the undersea section and tunnel linings, read our articles on India’s first undersea bullet train tunnel and concrete lining in tunnels.

Power and systems

The line uses a 2×25 kV compound catenary for traction power. About 22,000 steel overhead equipment (OHE) masts are needed, and more than 9,300 had been installed by September 2026. In late October 2026 NHSRCL also briefed the media on the corridor’s 14 traction substations. Signalling, rolling stock supply and testing still have to follow the civil work, and that is where most schedule risk now sits.

When will it open?

Official statements point to the first section, about 100 km between Surat and Vapi, opening in 2027. A Business Today report from August 2026 gave the target as August 2027. A piece written by the Railway Minister and published in October 2026 said the first service is scheduled to begin in the final quarter of 2027. The full corridor has been talked about for the end of 2029, but the Ministry says the date depends on civil, track, electrical, signalling and train supply work all coming together. Treat any single date with caution until the trains are on test.

What civil engineers can learn from MAHSR

• Industrialise the repetitive part. Full span launching turns a viaduct into a production line. The same idea applies to any project with many identical elements.
• Design for settlement, not just strength. Ballastless track forgives almost nothing, so foundation design is driven by long-term movement.
• Use the right method for each zone. One corridor uses full span, segmental, balanced cantilever, steel truss, TBM and NATM. Good engineering is choosing well between them.
• Logistics is engineering. Moving a 970 tonne girder safely along a finished deck is as much a design problem as the girder itself.

If you like large Indian rail projects, our explainers on the Chenab rail bridge and the Mumbai–Pune Missing Link tunnels cover two more.

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