Anyone who travels regularly by train will recognise the experience. The phone shows a mobile signal, sometimes even 5G, but webpages refuse to load, video calls freeze and messages remain stuck sending. Switching to the train’s Wi-Fi may not help either. The device connects to the access point, but there is little or no usable internet behind it.
Providing reliable connectivity on a moving train is much harder than providing coverage to a home, office or even a vehicle on a road. A railway combines weak and inconsistent outdoor coverage, a radio-unfriendly metal carriage, high-speed mobility, frequent handovers, difficult terrain and hundreds of passengers competing for limited capacity.
Recent measurement studies confirm how serious the problem remains.
An Ofcom-commissioned study measured mobile and on-board Wi-Fi performance across 50 journeys on 24 railway lines in Great Britain during February and March 2026. Its “Good Performance” threshold required at least 5 Mbit/s download, 1.5 Mbit/s upload and latency of no more than 50 milliseconds in the same test cycle.
The best-performing mobile network met that threshold only 42% of the time. The other three networks achieved between 17% and 21%. On-board Wi-Fi met the threshold just 1% of the time. Ofcom also found that latency was often the factor that caused a test to fail, even when download and upload speeds appeared adequate.
While this is a UK study, the problem is by no means unique to the UK. Ookla’s Q2 2025 comparison of train Wi-Fi across Europe and Asia found enormous differences between countries. Sweden achieved a median download speed of 64.58 Mbit/s, followed by Switzerland at 29.79 Mbit/s. At the other end of the scale, the UK achieved 1.09 Mbit/s and the Netherlands just 0.41 Mbit/s. The gap between the strongest and weakest markets was as much as 158 times.
The variation is important because it demonstrates that poor connectivity is not an unavoidable consequence of travelling by train. The engineering challenges are common, but the results depend heavily on how rail connectivity is planned, funded and operated.
Mobile networks are normally designed around where people live, work and spend most of their time. Sites are positioned and optimised to serve towns, cities, business areas and roads. Railway lines frequently pass through rural areas, cuttings, forests, valleys and other places where there may be little commercial demand beyond the passengers moving through.
A train may therefore spend much of its journey travelling along the edge of several mobile cells rather than through a strong, continuous coverage layer designed for the railway. A conventional mobile coverage map may show service in the surrounding area without accurately representing the signal available inside a carriage moving along the track.
Even where the outdoor signal is usable, it still needs to enter the train.
Modern rolling stock contains metal bodywork, foil-backed insulation and energy-efficient glazing. Low-emissivity windows often contain thin metallic coatings that reduce heat transfer but also reflect or absorb radio signals. The signal available outside the train can consequently be far stronger than the one reaching a passenger’s phone.
The uplink can be even more difficult. A mobile base station has relatively high transmit power, large antennas and sophisticated radio processing. A smartphone has limited power and a very small internal antenna. The phone must transmit back through the same coated windows and carriage structure that weakened the incoming signal.
This can explain why a passenger appears to have a reasonable signal or acceptable download speed but still struggles with video calls, file uploads, cloud applications and voice over IP.
Speed adds another layer of complexity. A phone or on-board communications gateway must continually measure nearby cells and hand the connection from one to another. At high speed, the device remains within each cell for less time and radio conditions can change significantly between a measurement and the handover itself.
Short interruptions may be almost invisible to a background download, but they can freeze a video call, interrupt a conversation or cause an application session to time out. Tunnels, deep cuttings, hills, buildings and vegetation can create additional sudden changes in signal quality.
Capacity is also shared. A single train can bring hundreds of connected devices into a mobile cell at almost the same moment. That cell may also serve a station, surrounding homes, businesses, road users and another train travelling in the opposite direction. During busy periods, this concentrated demand can increase latency, reduce throughput and create packet loss.
On-board Wi-Fi does not bypass all these challenges.
The passenger’s device connects to an access point inside the train, but the traffic must then pass through an on-board gateway and roof-mounted antennas before reaching an external network. Most train Wi-Fi therefore depends on the same public mobile coverage that passengers may already be struggling to use directly.
A strong Wi-Fi icon only confirms that the passenger has a good connection to the access point. It says nothing about the quality or capacity of the train-to-ground backhaul.
Legacy on-board equipment can make the problem worse. Ookla found that nearly two in five train Wi-Fi connections across the European markets studied still used Wi-Fi 4, while around 22% used the lower-capacity and more interference-prone 2.4 GHz band. Newer Wi-Fi generations and 5 GHz can improve performance within the carriage, but Ookla concluded that the external backhaul is generally the dominant bottleneck.
Simply adding 5G is not a complete answer either. 5G can provide more spectrum, greater capacity, improved latency and better radio efficiency. Ofcom found that 5G Standalone produced a substantial improvement where it was available, but its availability along the measured routes was inconsistent.
The railway still needs continuous outdoor coverage. The signal still needs to enter the train, capacity is still shared, handovers still need to work and the on-board equipment and backhaul must support the service. A 5G icon does not guarantee that every link in this chain is performing well.
There are several possible solutions, but none solves the whole problem independently.
The conventional mobile network can be improved by adding sites near railway coverage gaps, aiming antenna sectors along the track, using lower-frequency spectrum for reach and penetration, and adding capacity around busy stations and corridors.
Where conventional macro coverage is insufficient, dedicated trackside networks can provide closely spaced sites, directional antennas, engineered tunnel coverage and high-capacity fibre or microwave backhaul. This changes the planning objective from serving nearby towns and roads to serving the railway itself.
Trains can use high-performance roof-mounted antennas connected to intelligent on-board gateways. These systems can combine connections from multiple mobile operators and dynamically switch or bond links as conditions change. Genuine diversity is greatest where the operators use independent infrastructure rather than sharing the same underlying radio network.
Laser-treated, RF-permeable windows provide another approach. Fine patterns are removed from the metallic window coating, allowing more of the outdoor mobile signal to enter while retaining much of the window’s thermal performance. Tests cited by Ookla reported losses of approximately 20 to 30 dB from conventional coated windows, and Deutsche Bahn has announced a large-scale programme to treat tens of thousands of windows.
This is a passive solution that can benefit passengers using different networks and frequency bands. However, it only improves signal entry. It cannot create outdoor coverage or additional network capacity.
Active systems such as repeaters, distributed antenna systems and on-board small cells can bring mobile service directly into the carriage. These systems must be carefully engineered and coordinated with the mobile operators.
There is also an interesting interaction between repeaters and RF-permeable windows. A repeater requires sufficient isolation between its external donor antenna and the service antennas inside the train. If more of the amplified indoor signal escapes through treated windows and reaches the donor antenna, it can reduce isolation and create feedback, gain reduction or oscillation. Repeaters and treated windows can coexist, but they need to be designed and tested as one RF system.
For passenger Wi-Fi, the likely long-term architecture is a hybrid one. Modern Wi-Fi 6 or Wi-Fi 7 access points inside the carriage can connect to an intelligent gateway combining multiple cellular operators, dedicated trackside networks, station connectivity and low Earth orbit satellite backhaul.
LEO satellite systems such as Starlink and OneWeb are increasingly being considered as complementary links, particularly on rural and lightly served routes. They are not replacements for terrestrial connectivity because tunnels, deep cuttings, buildings and vegetation can obstruct the view of the sky. Rail-certified terminals must also meet demanding requirements for vibration, weather protection, power and electromagnetic compatibility. The most resilient approach is therefore to combine satellite and terrestrial links rather than depend entirely on either one.
Ultimately, there is no single fix for poor connectivity on trains. Reliable service requires the complete chain to work together: Railway-specific outdoor coverage + signal entry into the carriage + on-board distribution + sufficient backhaul + reliable mobility + capacity management + commercial coordination
Improving only one layer can leave another layer as the bottleneck. The train operator, rail infrastructure owner, mobile operators, rolling-stock manufacturer, connectivity provider, regulator and government all control different parts of the solution.
The video below explores these challenges and possible solutions in more detail.
Finally, for anyone working on passenger connectivity, train-to-ground communications or trackside infrastructure, TrainComms 2026 takes place in London on 18 and 19 November. The conference brings together train operators, rail infrastructure organisations, mobile and satellite connectivity providers, equipment vendors and other specialists to discuss on-board Wi-Fi, trackside wireless networks and emerging LEO satellite solutions. I will be attending again this year, so please do say hello if you are there.
Related Posts:
- Connectivity Technology Blog: Boosting 5G Coverage with Transparent Antennas and Shared Infrastructure
- The 3G4G Blog: A Different Approach for Mobile Network Densification
- Connectivity Technology Blog: Seamless 5G Connectivity Across Germany on Deutsche Bahn
- The 3G4G Blog: The Digital Railway supported by FRMCS
- Telecoms Infrastructure Blog: London Underground Mobile Network Infrastructure
- Telecoms Infrastructure Blog: Bringing Connectivity to Underground Rail Network
- Connectivity Technology Blog: Futuristic Glass Antenna by NTT Docomo and AGC
