How Is a Metro Built? An Overview of the Process
The short answer to how a metro is built is this: a metro line is delivered through seven core phases that follow one another on paper but overlap heavily on site — planning and alignment, geotechnical investigation, excavation and tunnelling, station construction, mechanical and electrical fit-out, track and signalling, and finally testing and commissioning. Although only an entrance structure and a few escalators are visible at street level, a metro project is in reality an integrated engineering system buried under the city and stretching for kilometres.
The sequence of these phases looks clean on a programme chart, but on the ground they run in parallel. A tunnel boring machine advances at one end while a station box is excavated at the other, and somewhere in between track is already being laid in completed tunnel. This concurrency puts the logic of the critical path at the heart of every metro project: a delay on a single work face can ripple through the whole line and push back the opening date.
What sets metro construction apart from other infrastructure work is the sheer density of engineering disciplines involved. Geotechnics, soft-ground tunnelling, reinforced concrete, structural steel, ventilation, fire safety, high- and low-voltage electrical works, track mechanics and signalling software all converge in the same corridor. A metro line is therefore less the product of a single contractor than of a coordinated engineering orchestra. Below we open up each phase of that process in turn.
Planning, Alignment and Feasibility
Every metro project begins with a needs analysis. Transport demand models forecast current and future ridership, and those figures dictate the capacity of the line, the spacing of stations and the frequency of trains. On a typical urban metro, stations sit 800 to 1,500 metres apart — a critical trade-off that balances accessibility against journey speed. Stations placed too close slow the service down, while stations too far apart lengthen the walk to the platform.
Alignment selection is as much an urban-planning decision as an engineering one. The route is optimised in both plan and profile, taking account of existing building foundations, utility networks, groundwater and heritage fabric. Horizontal curves are usually held to a minimum radius of around 300 metres, and vertical gradients are typically capped near 3 to 4 percent to protect passenger comfort and limit traction demand. It is at this stage that the line is committed to deep bored tunnel, shallow cut-and-cover, or an elevated viaduct.
The feasibility study brings together cost, programme, land acquisition, environmental impact and the funding model. Because metro investment carries a high cost per kilometre, decisions made here shape the entire life of the asset. A poor alignment or a gap in the ground data turns into costs that compound through later phases. This is why experienced firms read the geological and urban risk maps together from the very first table, rather than treating them as separate problems.
Geotechnical Surveys and Ground Investigation
There is a well-worn saying in metro engineering: it is the ground, not the machine, that drives the tunnel. Geotechnical investigation is therefore the project's most critical and least visible investment. Boreholes drilled along the alignment reveal the soil stratigraphy, rock quality, groundwater table and the swelling or settlement behaviour of the ground. On a typical line, boreholes are spaced 50 to 100 metres apart, and closer at stations and key crossings.
The samples recovered are tested in the laboratory to establish cohesion, the angle of internal friction, permeability and bearing capacity. These data directly determine which tunnelling method is appropriate. In competent rock, drill-and-blast or the NATM approach is economical; in water-saturated, loose or mixed ground, pressure-balanced TBM machines become almost mandatory. Choosing the wrong method is one of the most expensive mistakes in metro tunnel excavation.
Ground investigation is vital not only for the tunnels but also for the surface structures. The depth of station support walls, the length of bored piles, groundwater management and the settlement transmitted to neighbouring buildings are all calculated from this data. In a city centre, where a line runs beneath historic buildings, even millimetre-level settlement predictions matter. A thorough geotechnical campaign dramatically reduces the number of surprises on site, and with them the cost overrun.
Excavation and Tunnelling Methods: Cut-and-Cover, NATM and TBM
A metro tunnel is excavated by three principal methods, and most lines use all three. The cut-and-cover method suits shallow alignments: support walls of diaphragm panels or bored piles are installed first, the ground is excavated from the top down, the reinforced-concrete box is cast in place, and the surface is reinstated over it. It is usually economical for stations and shallow sections, but it disrupts surface traffic and city life for a long period.
For deep running tunnels, two methods dominate. NATM (the New Austrian Tunnelling Method) mobilises the ground's own load-bearing capacity: excavation proceeds in stages, immediate support is provided with sprayed concrete (shotcrete), steel mesh and rock bolts, and deformations are monitored continuously with instruments. Its flexibility lets it adapt to variable ground and to the wide cross-sections of station caverns. The TBM (tunnel boring machine), by contrast, is a full-face, mechanised and fast method; it lines the bore immediately with precast segmental rings and can balance the working face under pressure even in water-saturated ground.
Choosing the right method depends on the balance between ground conditions, tunnel length, depth, the density of surface development and the budget. Over long, homogeneous drives the TBM's advance rate of 10 to 15 metres a day justifies its cost, while NATM is more flexible on short stretches, variable cross-sections and crossover caverns. In practice, experienced contractors combine these methods on a single line — we examine each in detail in our dedicated articles on the NATM, TBM and cut-and-cover methods.
Station Construction: Structures Beneath the City
Stations are both the most expensive and the most complex structures on a metro line. A station typically comprises three layers: the entrance buildings at street level, the ticket and control hall (concourse) in the middle, and the platform level at the bottom. This volume is most often built by cut-and-cover as a deep reinforced-concrete box; on deep lines, large mined caverns excavated by NATM may instead house the platforms.
Station construction begins with the safety of the excavation support. Diaphragm walls or bored piles retain the ground, steel anchors and waling systems resist lateral pressures, and deep-well pumps bring groundwater under control. As the dig advances, intermediate slabs are built; this structural skeleton must reserve openings from the outset for the escalators, lifts, ventilation ducts and plant rooms that will later sit on it. One of the most common mistakes in station construction is failing to plan service routes adequately during the structural phase.
Stations are also the focal point of fire safety and evacuation scenarios. Smoke extraction shafts, pressurised escape stairs, two-directional egress routes and fire-rated compartments are factored in while the load-bearing system is still being designed. KMB Metro Altyapı's experience with station works on the Kiev metro and its involvement in the Dwarka Metro project in New Delhi shows that these structures are far more than concrete pours; they are a careful balancing act where architecture, engineering and operational requirements meet in a single volume.
MEP Systems, Track Laying and Signalling
Once the raw concrete shell is complete, the metro's truly living systems come into play. Mechanical and electrical (MEP) services are the invisible backbone that makes the metro operable: tunnel and station ventilation, smoke-control fans, fire detection and suppression, drainage pumps, lighting, high- and low-voltage distribution, escalators and lifts are all installed in this scope. Because these systems must communicate with one another and with the signalling, cable trays and ducts are routed through the openings reserved during the structural phase.
Next comes the permanent way, the track system. Metros generally favour ballastless (embedded or slab) track; it reduces vibration and noise, lowers maintenance, and saves space within the tunnel cross-section. Rails are aligned to a precise geometry, welded into continuous rail and fixed to the base with elastic fastening systems. Power is delivered to the train through a third rail or an overhead catenary; each solution has advantages tied to tunnel geometry and operating speed.
The final layer is the brain of the modern metro: the signalling and traffic-control system. Today most new lines are built for driverless or semi-automatic operation using CBTC (communications-based train control), in which trains automatically keep a safe distance from one another. This system optimises headways down to the order of seconds, allowing far more passengers to be carried over the same infrastructure. Commissioning the signalling is the most delicate and longest-testing stage of the entire line.
Testing, Commissioning and Opening to Service
A metro does not open the moment construction ends; ahead of it lies a testing and commissioning programme that runs for months. The process starts by verifying the subsystems one by one (static tests): electrical distribution, ventilation, fire systems and track geometry are checked separately. Dynamic tests follow; trains are first run along the line empty at progressively higher speeds, while braking distances, signalling responses and energy consumption are measured.
The most critical stage is integration testing. Here the train, signalling, power, communications and station systems are made to talk to one another under realistic scenarios. Emergency stops, fire evacuation, power loss and the recovery of a failed train are rehearsed again and again. No metro can begin carrying passengers until these tests are complete, because safety is the one item in metro operation that is never up for negotiation.
The final step is the trial run: the line is operated to the full timetable but without passengers for several weeks, so that staff are trained and the reliability of the system is proven. When an independent safety authority issues its certificate, the line officially opens. This long journey, from planning to the day of first service, makes plain why metro projects typically take years and why they demand experienced, multidisciplinary contractors.
Why Experience Is Decisive: The Role of the Multidisciplinary Contractor
Every phase of metro construction demands its own expertise, but the real difficulty lies in linking those expertises together without a break. When the connections fail — between geotechnical prediction and tunnelling method, between structural design and service routing, between track geometry and signalling software — delays and cost overruns begin. Metro work is therefore won not by individually strong disciplines so much as by contractors who can manage the coordination between them.
This is where institutional memory becomes decisive. A team that has driven tunnels in different ground, in different countries and by different methods senses surprises on site far earlier. KMB Metro Altyapı brings together more than 75 years of accumulated know-how, born of the partnership between Troy of Türkiye and Kyivmetrobud of Ukraine, spanning field experience from the Kiev metro stations to the Dwarka Metro line in New Delhi, and from the Voronezh railway tunnel to a range of dam and infrastructure projects. On projects with as little tolerance for error as a metro, that experience is the most tangible form of assurance.
In the end, although the technical answer to how a metro is built can be summarised in seven phases, what carries a project to success is the ability to run those phases along a single critical path without compromising on safety or quality. When the right ground investigation, the right tunnelling method and the right integration management come together, this complex system buried beneath the city becomes a backbone that carries millions of passengers safely for decades.