14 April 2025 | London, England

Telecoms.com periodically invites expert third parties to share their views on the industry’s most pressing issues. In this piece Luke Kehoe, Industry Analyst at Ookla, looks at the challenges and opportunities from installing mobile connectivity in underground rail networks. This article was originally published by Telcoms.com. View the article here.

In partnership with Boldyn Networks (Boldyn) and the four major UK mobile operators, Transport for London (TfL) is working to bring high-speed 4G and 5G mobile connectivity both underground and at street level across London.

Customers are already benefitting from 4G and 5G coverage in stations and tunnels on sections of the Jubilee, Central, Piccadilly, Victoria and Northern lines, as well as throughout the entire Elizabeth line. TfL aims to achieve uninterrupted mobile connectivity across 80% of the network’s stations this year, but faces significant challenges retrofitting mobile infrastructure within the world’s oldest underground system.

As the network buildout progresses, mobile connectivity across the Underground is steadily improving, though coverage still varies between lines. Deploying this infrastructure in such a complex environment presents significant challenges, which may not always be apparent to consumers.

The network buildout represents an unprecedented technical undertaking for TfL, Boldyn Networks and the UK’s mobile operators. This ambitious infrastructure project involves installing 2,000km of dedicated cabling above and below ground, deploying thousands of radio units and coordinating extensive engineering works during weeknight shifts—all while ensuring the continuous operation.

Global leaders in underground connectivity

London is not alone in its efforts to improve underground connectivity for passengers. Similar large-scale installation projects have been observed in other major cities around the world:

Hong Kong

• Since its opening in 1979, Hong Kong's Mass Transit Railway (MTR) has emerged as one of the world's busiest metro systems, serving over 5.5 million passengers on an average weekday. The MTR was the site of Boldyn's first comprehensive distributed antenna system (DAS) underground deployment, where the company collaborated with local mobile operators to establish an integrated communications infrastructure spanning private mobile radio, public safety networks, mobile coverage and Wi-Fi across the entire underground and light rail system.
• Boldyn recently collaborated with mobile operators to enhance 5G services on the MTR network, deploying advanced infrastructure in the 3.5 GHz (n78) frequency band. By implementing an innovative radiation cable structure and leveraging enhanced MIMO (Multiple Input, Multiple Output) technology, they significantly improved mobile data speeds across key sections of the network.
• Hong Kong's uniquely dense urban landscape presented significant challenges for the mobile deployments on the MTR. One particularly demanding installation on the West Island Line exemplified these difficulties: technicians were required to repeatedly scale a 20-storey site, often lugging test equipment up and down a steep slope multiple times a day to ensure comprehensive network coverage at an exit.

Spain

• Mobile coverage has been extensively deployed across metro systems in Spanish cities like Barcelona and Bilbao. Madrid, in particular, boasts one of Europe's most comprehensive underground mobile network buildouts, with full connectivity spanning its near 300km metro system.
• At the turn of the millennium, Metrocall emerged as a pioneering neutral host operator, established by Metro de Madrid to develop comprehensive mobile infrastructure for the underground. Four years ago, it completed a comprehensive upgrade of Metro de Madrid’s mobile infrastructure, extending 4G coverage across its entire network of over 240 stations and all transit lines.
• In 2020, Cellnex—Europe's largest tower company and a key developer of metro mobile networks in cities like Milan and Brescia—acquired a 60% stake in Metrocall, subsequently investing in expanding 5G coverage throughout the system.

United States

• Mirroring its work on the London Underground, Boldyn has also collaborated with New York City's Metropolitan Transportation Authority (MTA) to provide comprehensive 5G coverage for customers of AT&T, T-Mobile, and Verizon. The project has deployed mobile infrastructure throughout 418 track miles of subway tunnels, alongside implementing an enhanced free Wi-Fi service across all 191 above-ground subway stations and the 21 Staten Island Railway stations.
• Late last year, 5G services were activated throughout the 42nd Street Shuttle line, one of the system's most heavily travelled tunnels connecting Times Square to Grand Central. This route became the first in Manhattan to receive comprehensive mobile coverage. Complementing the subway network deployments, the city is expanding the LinkNYC network—digital kiosks providing public information, device charging, and emergency calling services—supported by increased fibre backhaul infrastructure enabled through the subway network deployments.

Challenges in deployment

Installing underground networks requires balancing legacy infrastructure, both the tunnels themselves and any existing network infrastructure, managing installation alongside near constant use, and of course, the cost, to deliver a better-connected experience to passengers.

Metro systems built decades ago, like those in New York and London, require complex retrofitting to support advanced connectivity. In metro systems where 2G, 3G and 4G infrastructure is already in place, upgrading to 5G and deploying higher frequency spectrum in the 2.6 and 3.5 GHz bands typically requires all-new leaky feeder cable and small cell antenna systems.

City administrations and infrastructure companies aiming to deploy enhanced connectivity options also encounter significant cost and time challenges when installing mobile and fibre infrastructure in underground tunnels. Newer 5G systems compound these challenges, as higher frequency bands demonstrate poorer signal propagation and require additional antennas to maintain equivalent coverage levels.

In addition to the time and infrastructure challenges, underground environments also present significant challenges for signal transmission, especially in higher frequency bands like 3.5 GHz. Radio engineers must address signal attenuation caused by dense metal carriages designed for sound proofing and mitigate the Doppler effect resulting from increasingly high-speed train movements. On extended track segments, slim form factor radios installed within tunnels are increasingly necessary to ensure consistent mobile coverage.

And the most basic problem of all? Working around the millions of users who rely on these networks every day. Using the London Underground as an example, with an average of 3.23 million daily users, 1.2 billion annual passengers, and five lines operating 24 hours during weekends, network installation across entire lines presents a significant logistical challenge. Most installation work must be completed within a four-hour window, potentially spanning up to 30 stations simultaneously.

The track to seamless mobile coverage on the London Underground

At the heart of the London Underground connectivity rollout is an innovative 20-year concession agreement between Boldyn and TfL to design, build and operate the shared mobile network across the metro system.

The concession is structured to be financially sustainable for TfL, with Boldyn wholly responsible for the capital investment required to develop the network, which is expected to exceed £1 billion over the lifetime of the project. Under this model, similar to the previous examples in places like Hong Kong and New York, Boldyn acts as a neutral host operator, leasing the infrastructure to operators, thereby minimising the risk profile for them. 

As Boldyn progressively activates its infrastructure across the Underground on a station-by-station basis, with some exceptions (e.g., Elizabeth Line), it then turns to mobile operators to enable their services and make decisions regarding spectrum configuration and technology deployment.

In addition to providing mobile coverage to customers of the UK's mobile operators, Boldyn’s infrastructure also facilitates the Emergency Services Network (ESN) as a fifth operator. The ESN is a 4G-based network operated by EE on the 800 MHz band, ensuring first responders have priority access to communications during emergency situations.

Delivering mobile connectivity to the London Underground then, is not simply to enable passengers to stay connected while traveling and improve the overall passenger journey, but to ensure the safety of its passengers too. The agreement between Boldyn and TfL marks a crucial step towards ensuring both, but requires continued strategic planning, and significant labour and investment from all parties involved.