The modern metro is not just a transport artery, but a complex digital organism permeated with radio waves. For most passengers Wi-Fi in the metro It's a familiar comfort, allowing people to while away the time while traveling by watching videos or chatting. However, few people consider the colossal amount of engineering and sophisticated equipment that goes into maintaining a stable connection tens of meters underground.
Unlike home internet, where the signal propagates through relatively free space, underground utility lines present an extreme environment for radio waves. Concrete walls, metal train structures, and high passenger density create unique conditions that require specialized solutions. In this article, we'll take a detailed look at network architecture, equipment operation principles, and the reasons why connection may suddenly drop.
The main goal of the underground network is to ensure continuous coverage along the entire train route. This is achieved through dedicated cable systems and access points that work in conjunction with equipment on the rolling stock. Understanding these processes helps us understand why internet service is excellent in some sections of the tunnel and completely absent in others.
Underground network architecture and base stations
The foundation of any cellular or Wi-Fi network in the metro are base stations located along the tracks. In tunnels, it's impossible to use conventional outdoor antennas, as the concrete and soil completely block the signal from the surface. Therefore, telecom operators deploy a distributed antenna system (DAS) that extends along the entire track. Base stations In the metro, they are often located in separate technical rooms, and the signal is transmitted to the antennas via fiber optics.
Antennas are placed at a specific spacing, typically every 150–200 meters, to ensure coverage overlap. This is necessary for the technology to be implemented. handover Handover is the process of switching a passenger's device from one antenna to another without interrupting the connection. When the train is moving at 80 km/h, this process must occur within milliseconds, otherwise the video conference will be interrupted.
⚠️ Please note: Equipment configuration and frequency ranges used may vary depending on the city and year of metro line modernization.
A particularly challenging task is a station where a huge number of people congregate. Here, the subscriber density per base station increases hundreds of times compared to a section of the station. To solve this problem, sector antennas and small cells, which divide the space into small cells, distributing the load between them.
The problem of signal propagation in a tunnel
From a radio engineering perspective, a subway tunnel is a waveguide, but a waveguide with extremely complex characteristics. Metal rails, a contact third rail, and smooth walls create multiple signal reflections. This phenomenon is known as multipath propagation, leads to the fact that waves arriving at the receiver via different paths can cancel each other out.
Furthermore, the train car itself is a Faraday cage. The metal casing effectively shields the interior from external electromagnetic fields. This is why, without special antennas mounted on the roof of the train, getting a stable Wi-Fi connection inside the car is virtually impossible. The signal simply cannot penetrate the thick layer of metal.
Why does the signal disappear when turning?
On straight sections, the tunnel acts like a pipe, directing the signal. However, on sharp turns, the wave propagation geometry changes dramatically, and the antennas may lose line of sight, causing brief connection interruptions.
To combat signal attenuation, the following are used: radiating cablesThis is a coaxial cable with a specially slitted outer sheath. It acts as a long antenna, radiating the signal evenly along its entire length. This approach eliminates "dead zones" between access points.
- 📶 Free space attenuation — the signal loses energy with distance.
- 🏗️ Shielding - concrete and reinforcement absorb radio waves.
- 🚇 Faraday cage effect — the car body blocks external signals.
- 🔄 Interference — superposition of reflected waves on each other.
Wi-Fi technology on board rolling stock
To enable passengers to use the internet, special equipment is installed on the roof of each carriage or at the head of the train. This modem router, equipped with external antennas that receive a signal from the metro infrastructure. Inside the car, this signal is retransmitted via a local Wi-Fi network.
The onboard system operates in two directions: it receives data from base stations and transmits requests from passenger devices. As the train moves, the roof antenna must constantly switch between ground infrastructure cells. Modern systems use technologies MIMO (Multiple Input Multiple Output), which allows multiple data streams to be transmitted simultaneously through different antennas, significantly increasing speed.
Data buffering is an important element. When passing through areas with unstable coverage or during sudden switching between base stations, onboard equipment can briefly pause packet transmission, only to resend them when the connection is restored. This creates the illusion of a continuous data flow for the user.
Onboard equipment must be extremely reliable and resistant to vibration, temperature fluctuations, and electromagnetic interference from the traction motors. Any malfunction of the roof antenna will result in loss of communication throughout the entire train.
Roaming and switching between base stations
One of the most challenging aspects of setting up Wi-Fi in the metro is ensuring seamless roaming. When a train travels at 80 km/h, it covers a distance of 20 meters in less than a second. During this time, a passenger's device must disconnect from one access point and connect to the next.
The switching process, or handover, requires the exchange of service packets between the client device and the network. If this process takes too long, the TCP connection (used to load pages and videos) may be broken, and you'll have to reopen the page. Metro uses fast roaming protocols such as 802.11r, which reduce reconnection time to a minimum.
Adding to the complexity is the fact that a single tunnel can contain signals from dozens of access points simultaneously. The device must correctly identify the most suitable one for connection, ignoring weaker or more congested signals. Algorithms for selecting the best access point operate on both the client (your smartphone) and the network.
When a train stops at a station, the network load increases dramatically. Thousands of passengers simultaneously try to send messages or update social media feeds. The network must dynamically redistribute resources to ensure no user is left without access, although speeds for each individual subscriber may drop during peak hours.
Why the connection is lost: the main reasons
Despite advanced technology, users often experience connection loss. The most common cause is physical signal blockage. When a train enters a depot or passes through sections where the infrastructure has not yet been upgraded, coverage may be completely lost. The signal can also disappear in areas with complex tunnel geometry.
The second reason is network congestion. During peak hours, the number of connected devices can reach into the thousands per train. Even powerful equipment has a bandwidth limit. When the limit is reached, new requests are simply queued or dropped, which you see as application freezing.
⚠️ Please note: Technical work on communication networks can be carried out at night, but sometimes emergency situations require intervention during the day, which leads to a temporary lack of signal in certain areas.
The third factor is interference. The metro employs numerous electronic systems: signaling, driver communications, and video surveillance. Although they operate on different frequencies, powerful pulsed interference from overhead lines or engines can temporarily jam Wi-Fi channels, especially in the 2.4 GHz band.
| Reason for failure | Description of the problem | Frequency of occurrence |
|---|---|---|
| Tunnel effect | Signal loss in areas without coverage | Rarely (on old lines) |
| Network congestion | Exceeding the number of subscribers per cell | Frequently (rush hours) |
| Handover failure | Antenna switching error | Average (at high speeds) |
| Electromagnetic interference | The influence of the traction network on the radio channel | Rarely |
Data security on public Wi-Fi
Using open Wi-Fi networks in the metro carries certain security risks. Since traffic is transmitted over radio waves, it could theoretically be intercepted by nearby attackers. Although modern encryption protocols (HTTPS) protect website content, metadata about the resources you visit may be visible.
Subway operators are implementing authentication systems, often requiring a phone number or an app. This is not only a user identification method but also a security feature that helps filter out bots and potential attackers. However, relying solely on the operator's network is not recommended.
To protect your confidential information, it is recommended to use VPN connection (Virtual Private Network). This will create a secure tunnel between your device and the server, rendering intercepted data useless to an attacker. You should also disable the automatic connection to known networks feature to prevent your phone from attempting to connect to fake access points with similar names.
☑️ Safety rules for public Wi-Fi
If you need to conduct a banking transaction, it is better to switch to 4G/5G mobile internet, which has a higher level of encryption at the cellular operator level.
Development Prospects: 5G and New Standards
The future of communications in the metro is inextricably linked with the introduction of fifth-generation networks (5G). This standard promises not just increased speed, but a fundamental change in network architecture. The high frequency of the 5G signal allows for the transmission of enormous amounts of data, but requires a much denser deployment of base stations.
The introduction of 5G in the metro will enable the "train-to-ground" concept with real-time telemetry transmission, improving ride safety. For passengers, this means the ability to watch 4K/8K video without buffering, even in a crowded train. However, this will require equipment upgrades on all lines.
Li-Fi technologies, which transmit data via light waves, are also being considered. In metro environments, where lighting is ubiquitous, this could be an excellent complement to radio, reducing airtime and ensuring stable connectivity in crowded areas.