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This is not TfL's first foray into improving internet connectivity on London's railways. In 2017, the company tested wireless network distribution technology on the Waterloo and City line, which enabled it to practice laying cables for its wider connectivity plans. Though 4G has not yet been fully implemented, WiFi is now available at over 260 tube and 79 London Overground stations across the network, thanks to a partnership with Virgin Media.

Connectivity is still limited, passengers can only gain access to WiFi while standing on platforms, and only after following a slow and cumbersome sign-up process. After they enter a train and head into a tunnel, the connection drops. Having 4G mobile connectivity in the tunnels would therefore allow customers to check their emails, read the latest news and social media updates and check for live travel information while underground.

Alongside all the obvious browsing benefits, this would potentially facilitate much easier journeys for passengers. For example, they could look up real-time updates on the status of lines and use different routes in the event of a signal failure or overcrowding. Another potential boost will be passenger safety, which has been a key consideration of other metro networks. For example, Toronto’s metro has launched its SafeTTC mobile app, which offers Toronto Transit Commission (TTC) passengers a discreet and quick way of reporting harassment, safety concerns or suspicious activity directly to a transit control centre.

From TfL’s perspective, 4G mobile connectivity will also be key to its deployment of an enhanced Emergency Services Network (ESN), which supports communications between ambulance, fire and police services in stations and tunnels.

According to Real Wireless COO Oliver Bosshard, ESN is a fantastic enabler for overall mobile connectivity on the underground, as many parts of the dedicated public mobile network and ESN can be shared. Sharing brings cost benefits and can reduce the overall complexity of providing a network for first responders and a public mobile network.

“Mobile coverage and good underground connectivity would also mean that TfL would be less reliant on other kinds of operational communication technologies,” says Bosshard. “It enables TfL and other parties to develop and test new applications, some of which have not been thought of today. Most of these applications will run over the top of a suitable digital network – in a way not dissimilar to apps on smartphones.”

An obstacle to 4G mobile connectivity taking off in the past has been the business case. The question ‘what’s in it for us?’ extends from TfL to the mobile network operators who will support the project, not to mention the selection of companies that own the trains running on tracks and infrastructure.

“Several parties could benefit from wireless connectivity, but there is usually not enough commercial incentive – or benefit – for a single party to lead and fund such a system,” says Bosshard. “This creates a significant challenge to achieving mobile connectivity and is likely to be one of the main reasons why the underground in London is still without mobile coverage despite numerous attempts over the last 15 years. But a shared network, benefiting multiple parties by addressing their commercial, operational or social needs, has the best chance to succeed and justify a joint investment.”

“Any mobile communications solution for the underground is – because of the environment – going to be extremely expensive to deploy and operate. But it is unlikely to create significant incremental revenue for mobile network operators to be able to justify their investment. For them, being part of the project may be more about improving their image rather than increasing revenues.”

On the other hand, TfL will be able to compel mobile network operators such as EE or Three to pay access fees. In addition, the potential benefits of tracking connected passenger data have also borne fruit. Late last year, TfL revealed a major project to monitor passengers’ mobile phones on the London Underground to better understand travel patterns, as well as boost advertising revenue by up to £322m.

In order to effectively provide mobile connectivity to the underground, the winning TfL contractor could install a range of different technologies. According to the TfL spokesperson, they would need to implement neutral host mobile coverage on below ground tunnels, stations and platforms of the London Underground and the Elizabeth line, as well as constructing a commercial fibre optic network.

However, it’s clear that an installation of any kind will prove difficult. Around 45% of the London Underground rail network exists in narrow and deep tunnels, some of which were built in Victorian times. Radio signals are unable to penetrate subterranean spaces, as well as heavy construction materials such as concrete and brick. Changes in track elevation and curves that radio waves are unable to bend around present additional challenges.

"There is a risk that TfL may not allow the installation of active radio kit in the tunnels due to limited access, safety concerns, heat generation or because there's not enough space or power,” says Bosshard. “If that’s going to happen, that makes coverage for the longer tunnels harder. However, it could still work if lower frequencies on an optimised radiating cable are used, but that would come at a cost of lower capacity for the longer tunnels.”

What’s more, any systems that are installed will need to be resistant to brake dust released by friction between brake pads, wheels and rails. This steel-based dust is conductive, making it a particular issue for sensitive electronic equipment. Another issue is access, which is becoming increasingly difficult as the tube takes steps towards 24-hour services. With 4.8 million passengers using the London Underground daily, the logistics of installation become increasingly complicated.

Systems introduced will need to fit into small spaces and require little to no maintenance. However, any solution will also need to be able to support multiple frequencies for both the public network and ESN, and support future upgrades without significant additional investment. This will become particularly important given the rollout of next-generation services, such as 5G.

According to Bosshard, the scale of the project, the need for comprehensive safety cases and the skills required to design, deploy and optimise such a large distributed antenna system-based network, will make a full deployment by 2019 a difficult target for TfL to achieve. However, after years of trailing behind other metros in this area, the “will” for a fully connected London Underground has never been stronger.