FRMCS is redefining what underground rail communications networks need to deliver. This example shows why operators must now plan for far more than continuous coverage—balancing capacity, mobility, resilience, and long-term scalability in one future-ready RF strategy.
Consider a metropolitan underground railway with 25 km of twin-bore tunnels, 18 underground stations, and operating speeds of up to 80 km/h. The existing operational radio system is GSM-R over leaky feeder cable, while passenger cellular and Wi-Fi services already operate throughout stations. ETCS Level 2 is planned within the next decade, with Future Railway Mobile Communication System (FRMCS) forming the communications platform that will support the railway's digital transformation.
From an RF planning perspective, this is a highly constrained mixed-use environment. Tunnel waveguide effects, highly reflective station spaces, legacy radiating cable infrastructure, and multiple coexisting wireless services all influence radio performance. Designing a network that supports both today's operational requirements and tomorrow's digital railway demands requires far more than maintaining continuous coverage.
Under GSM-R, railway communications focused primarily on delivering reliable voice and signalling services, with relatively limited data requirements. FRMCS fundamentally changes that model.
As the 5G-based successor to GSM-R, FRMCS is expected to support:
This shift means railway operators and engineering partners must now design underground radio networks that deliver not only reliable coverage, but also predictable mobility, sufficient capacity, low latency, resilience, and long-term scalability as part of a single communications strategy.
Returning to our example railway, the existing leaky feeder system already provides reliable longitudinal coverage throughout the tunnels. Naturally, operators want to understand whether that investment can continue supporting the next generation of railway communications.
The answer is often yes - but not on its own.
Radiating cable continues to offer strong value for predictable longitudinal coverage. However, future service demand introduces much tougher requirements around capacity, sector granularity, isolation, and the practical delivery of higher-order multiple-input and multiple-output (MIMO) performance.
For operators planning migration, this is where intelligent design decisions become critical. Rather than viewing migration as a simple choice between retaining or replacing existing infrastructure, many operators are evaluating hybrid architectures. Retaining leaky feeder through tunnel sections, where continuous coverage remains operationally advantageous, while deploying Distributed Antenna Systems (DAS) within stations—where density, service overlap and sector control place greater demands on the network—can provide an effective migration path that balances infrastructure reuse with long-term scalability.
Evaluating these design options requires more than simplified propagation calculations. Using Ranplan Professional, engineers can create an accurate 3D digital twin of tunnels, stations and the surrounding environment. By accurately representing tunnel geometry, infrastructure and material characteristics, the digital twin allows engineers to evaluate design decisions before physical deployment. They can then compare alternative architectures, predict propagation behaviour, analyse interference, and validate capacity before deployment decisions are made.
As trains approach underground stations, the radio environment changes rapidly. Tunnel curvature, changing reflection characteristics, and overlapping sectors all influence serving cell selection and handover performance.
Network simulations may show sector-to-sector handovers with interruption characteristics that remain acceptable for GSM-R voice services but become unacceptable for continuous IP-based ETCS data sessions or future ATO applications.
This is where FRMCS design fundamentally differs from GSM-R. Cell-edge behaviour, neighbour relations, trigger tuning, and overlap geometry are no longer post-deployment optimisation tasks; they are fundamental design considerations that determine whether operational services remain uninterrupted throughout the journey.
Using advanced simulation within Ranplan Professional, engineers can evaluate train movement, handover behaviour and service continuity before deployment, allowing mobility issues to be identified and resolved during the design stage rather than after installation.
Capacity planning also changes fundamentally. A GSM-R network could often be dimensioned around a relatively small operational traffic profile, with limited expectation of concurrent high-rate services. FRMCS demands a more strategic and future-focused methodology. It must accommodate continuous train-to-ground telemetry, ETCS packet flows, operational CCTV, maintenance communications, condition monitoring sensors, and future digital railway applications operating simultaneously.
The question is no longer whether the network performs on the day it is commissioned, but whether the chosen architecture continues to support increasing operational demand throughout the next twenty to thirty years.
Using realistic traffic models and advanced RF simulation within Ranplan Professional, planners can evaluate future demand, identify capacity bottlenecks and dimension networks with sufficient headroom to accommodate future growth without wholesale redesign of the RF layer. For rail specialists, this is a central migration principle: FRMCS should be deployed as an extensible communications platform, not as a narrowly optimised replacement for GSM-R.
Coverage and capacity alone do not guarantee operational performance. Underground rail networks must remain resilient during equipment failures, planned maintenance and changing operational conditions. This requires careful planning of redundant coverage, overlapping sectors and resilient infrastructure to maintain mission-critical communications throughout the network lifecycle.
At the same time, FRMCS must coexist alongside passenger cellular networks, Wi-Fi and other operational radio systems. Understanding how these services interact—and where interference may occur—is an increasingly important part of modern RF planning.
By modelling these scenarios within a digital twin, engineers gain greater confidence that network performance can be maintained under both normal operating conditions and unexpected events.
The industry lesson is clear. Migration from GSM-R to FRMCS is not a like-for-like radio replacement; it is a strategic redesign of the communications platform that will underpin railway operations for decades.
The question is no longer simply, "Can we maintain coverage?"
The real question becomes: "Can this radio architecture reliably support deterministic, safety-related and operational IP services within one of the most challenging RF environments for the next 20 to 30 years?"
Answering that question requires accurate environmental modelling, advanced propagation analysis and realistic simulation that allow infrastructure decisions to be validated before deployment begins.
With Ranplan Professional, engineers can create high-fidelity digital twins, evaluate infrastructure reuse, analyse mobility, predict capacity and optimise network performance throughout the design process - reducing project risk while delivering more resilient, future-ready underground communications networks.
That is the real challenge behind FRMCS migration—and where advanced RF planning and design become critical to unlocking the full value of the digital railway.
As this example demonstrates, designing underground FRMCS networks involves far more than achieving continuous coverage. Every design decision—from infrastructure reuse and mobility to capacity and resilience—affects long-term operational performance.
For a deeper look at tunnel propagation, mobility, infrastructure selection, capacity planning and RF design best practices, download our Wireless Connectivity in Tunnels: A Practical Network Design Guide. It explores these topics in greater detail, with practical guidance and examples showing how Ranplan Professional supports the design, simulation and optimisation of wireless networks for tunnels and other complex underground environments.