For a city like Madrid and its metropolitan area, which is home to 7.3 million people and constitutes Spain’s largest conurbation, underground infrastructure such as the metro, commuter trains, walkways and transfer stations play a vital role in ensuring that the transport system is able to flow. Without them, there would be insufficient space at surface level to absorb the volume of passengers. The metro alone carries 677 million people per year, according to data for 2019.

Of the 300-plus stations in the network, Sol is the busiest by a significant distance, with 24.4 million passengers in 2019. And that is not its only record: it is also the oldest station, as it was the starting-point of the city’s first metro line, Sol-Cuatro Caminos, which opened in 1919. Following major extension work in 2009, Sol boasts the world’s largest station cavern, measuring over 200 metres long by 20 metres wide and 15 metres high.

TUNNEL INCLINE. The image is a cross-section of the tunnel, showing the 12-metre incline between Sol commuter station and Gran Vía metro station. Accessibility is provided by four moving walkways.

After this expansion, which followed the entry into service of the new tunnel between the two major railway stations of Atocha (to the south) and Chamartín (to the north), Sol was reopened as a transfer station, with access to three metro lines and two commuter lines (located at the deepest level). Here, at the end of the vast station cavern, a pedestrian tunnel was built in order to provide a link to another of Madrid’s historic stations, Gran Vía, located a little over 100 metres away beneath Montera street. After the initial structure was built, the tunnel remained closed while work was carried out on the metro and commuter branch lines.

Working on behalf of Adif, Ineco drew up the plans and supervised the work to prepare the tunnel for its entry into service. Work began in 2018 and recommenced in early 2021, while the Community of Madrid finalised the expansion of Gran Vía station. Work on the two projects was coordinated, and both facilities were opened at the same time in July 2021. The new pedestrian tunnel will increase the flow of passengers to a station that already ranks among the busiest. Madrid Metro estimates that the station will receive an additional 22,000 users per day, in addition to the 44,000 who already do so.

Key characteristics of a deep-lying site

In 2009, during the renovation of Sol station, the tunnel was bored out and a series of arched reinforced concrete sections installed, totalling 120 metres long by 5.7 metres wide. The route runs parallel to Calle Montera, above the railway tunnel, and ascends by approximately 12 metres on its way towards Gran Vía.

The plan drawn up by Ineco in 2018 detailed all of the actions required to make the tunnel operational: clad the concrete walls with vandal-proof materials; install all of the systems for lighting, ventilation, security, fire-fighting, communications, signalling, access control and ticket machines at the entrance to Gran Vía station; and install four large moving walkways to overcome the incline. Adif also commissioned Ineco to provide site management services and coordinate health and safety for the construction work, which was carried out by Tragsa.

As well as coordination with other major projects that were undergoing implementation, these works had the added complexity of being carried out 20 metres underground. Not only that, but they were done while Sol station remained fully operational. Consequently, in order to transport the moving walkways (which were originally going to be brought from Gran Vía), draisines –track-based transport vehicles (1)– were used to carry the walkways to the platforms in sections (2). There, they were hoisted up to the mezzanine (the intermediate floor above the platforms, where the tunnel is located) using a block and tackle system (3), and then transferred into the tunnel using rolling platforms (4 and 5) and placed inside the excavated pits (6). These operations required meticulous planning and great precision in order to avoid damaging the station, and were carried out at night so that they did not interfere with normal operation.

POSITIONING THE WALKWAYS.

Another notable innovation is that, owing to its unique layout and design, the tunnel can be used as an evacuation route in the event of an emergency. In coordination with Renfe, Adif and the Madrid Metro, evacuation signalling has been installed in accordance with a specially designed and coordinated fire-protection and ventilation system, which means the tunnel can be used in a number of different emergency scenarios depending on where the emergency originates.

The development of modern Madrid is closely linked to that of its railway infrastructure. The increase in population forced the city to expand northwards at the beginning of the 20th century, and the construction of new stations, lines and railway connections were planned and implemented at the same time. Today, the capital’s main urban axis runs between the two major stations, Chamartín in the north and Atocha in the south, connected on the surface by the Prado, Recoletos and Castellana promenades and underground by three tunnels: two for commuter trains and one for high-speed trains, which has not yet opened.

Of the three tunnels, Recoletos was the first to be opened, in 1967, at the same time that the city was growing along the new urban corridor. With the opening of the subway, which had two stops –Recoletos and Nuevos Ministerios– development began on what starting the 80s would be Madrid’s commuter rail network, the largest in the country, which today carries more than 900,000 passengers every day.

The Recoletos tunnel is still the busiest in the country today: 470 trains and 200,000 passengers pass through it each work day

Recoletos is still the busiest railway tunnel in the country today: 470 trains and 200,000 passengers pass through it every work day, for a total of almost 3,300 journeys each week. 98% of this traffic corresponds to the Madrid commuter rail lines C-1, C-2, C-7, C-8 and C-10 –the rest runs through the Sol tunnel– along with some twenty medium- and long-distance trains per day.

Although improvement works were carried out in 2008, 2009 and 2012, the intensive use of this infrastructure after more than half a century in service made it necessary to undertake a more thorough renovation of the underground system. On behalf of Adif, Ineco provided the project and management of the works, as well as technical assistance, which required the closure of the tunnel between June and November 2019; on 17 November, the 7.3 kilometres tunnel was reopened to traffic.

Recoletos Station. / PHOTO_ELVIRA VILA (INECO)

The works were carried out against the clock in order to minimise the impact on Madrid’s railway network, which began operations in the mid-19th century. The first railway line in the capital, initially for the exclusive use of the Royal Family, linked Madrid with the Royal Palace of Aranjuez and was opened in 1851. It started from a stop (or ‘boarding platform’ as it was known at the time) that would later become the Mediodía station, today’s Atocha station.

The development of the railway parallels the growth of the city, which until the end of the 18th century was enclosed within walls with their corresponding puertas or ‘gates’, with the Puerta de Alcalá and Puerta de Toledo, for example, surviving up to the present day. The last wall, built by King Felipe IV, was demolished in 1868, making it possible to expand the city.

The TUNNEL layout follows the main urban corridor of Madrid, from Atocha to Chamartín, under the great boulevards of the capital

The first urban development plans, at the end of the 1920s, proposed growth along a large new north-south avenue that would structure the city, the Paseo de la Castellana. This planning included, among other installations, a new railway network of which the Recoletos tunnel was a part, which was designed and planned in 1933 following the same alignment of the future Paseo de la Castellana. The outbreak of the Spanish Civil War (1936-1939) and the economic difficulties of the post-war period paralysed these and many other projects for years, including the underground, which finally opened four decades later.

At the beginning of the new millennium, the growth in the demand for transport drove the expansion of the commuter rail network: In 2008, a second tunnel, Atocha-Sol-Nuevos Ministerios-Chamartín, was opened. A third tunnel, also between Atocha and Chamartín, which has already been completed, will be dedicated exclusively to high-speed trains, connecting all of the lines in the network.

ATOCHA (94,5M passengers/year*)

Built on the former Mediodía station, the existing railway complex, the largest in Spain, was opened in 1992 along with the Madrid-Seville high-speed line. It consists of two stations, the commuter station and the high-speed station. It is located in the Glorieta del Emperador Carlos V, where the Paseo del Prado begins and continues north to the Plaza de Colón. The three major national museums are located in this area: the Prado, the Museo Nacional Centro de Arte Reina Sofía and the Thyssen Museum.

Atocha Station. / PHOTO_ELVIRA VILA (INECO)

RECOLETOS (9,3M passengers/year*)

In the 16th and 17th centuries, this was an area with orchards known as Prado de Recoletos, in reference to the convent of Augustinian friars, located on the site now occupied by the National Library and the Archaeological Museum. The Paseo de Recoletos begins at the Plaza de Cibeles – with its famous fountain, a symbol of Madrid, and which houses, among others, the Palacio de Telecomunicaciones and headquarters of the City Council – and ends at Colón. In the 1960s and 1970s many old palaces and buildings were demolished and replaced by modern buildings.

Recoletos Station. / PHOTO_ELVIRA VILA (INECO)

NUEVOS MINISTERIOS (35,1M passengers/year*)

In the 1930s the city’s growth towards the north was first planned, along a large avenue, the Paseo de la Castellana, which opened to traffic in 1952, and Azca, a new residential, commercial and entertainment area. The Nuevos Ministerios complex was built alongside it and the first underground suburban station was opened here in 1967, located under the central courtyard. After several extensions, it has now become a large interchange, connecting to three metro lines and seven suburban lines.

Nuevos Ministerios Station. / PHOTO_ELVIRA VILA (INECO)

CHAMARTÍN (24,2M passengers/year*)

The route of the tunnel is separated from the axis of the Paseo de la Castellana until it reaches this station, which gets its name from the former village of Chamartín de la Rosa on which it is located. The first station was opened in 1967, and eight years later the new railway terminal was designed by the architects Alonso, Corrales and Molezún, along with the engineer Rafael Olaquiaga. In 2008, it was renovated to adapt to high-speed trains and connect to the new Sol tunnel. The next major remodelling has already begun with a view to the commissioning of the third exclusive high-speed tunnel between Atocha and Chamartín.

Chamartín Station. / PHOTO_ELVIRA VILA (INECO)

Track and overhead line renovation: The making-of step by step

1. To renovate the track, first the old ballast was removed (stripped) and collected on a conveyor train. In total, 15 km were cleared. 2. The ballasted track was lifted and replaced by slab track. The picture shows the new bi-block sleepers, type BP-SO, ready for the pouring of the slab track. 3. The concrete was poured directly in order to create the slab. In total, 23,000 m3 of reinforced concrete was used. 4. Machinery for the correct positioning of the track. Throughout the tunnel, 29,400 m of rails were installed in 288-metre long bars. 5. Pouring of track 2 from track 1, already completed, with a three-tank train. 6. Additional work carried out included the installation of 25 new track devices: 7 turnouts, 8 crossovers and 1 bretelle. In the image, at the entrance of Nuevos Ministerios, one of the turnouts that has already been assembled; in the tunnel vault, supports for the new rigid overhead line. 7. Assembly of part of the 1,300 rigid catenary bars installed in the tunnel. 8. The completed track, with the rigid overhead line already installed. 9. Running of new signals, wiring and signalling elements. 10. Electronic signalling control points were installed in the Recoletos and Nuevos Ministerios stations, upgrading the existing technology to a safer system.