Why CBTC on private 5G matters now

CAF and Cellnex have validated CAF’s OPTIO CBTC over a private 5G network, signaling a shift in how mission-critical rail control systems can be delivered and governed.

OPTIO CBTC validated over private 5G: lab and tunnel results

CAF’s signalling division and Cellnex demonstrated that OPTIO, a modular and multi-bearer CBTC platform, operates reliably on a private 5G network in both lab and field conditions, including challenging scenarios such as tunnels. The system already supports Wi‑Fi and LTE; adding 5G confirms a multi-access design that lets operators choose the right bearer per line, phase, or location. Private 5G brings ultra-low latency, higher capacity, stronger QoS control, and end-to-end security under the operator’s domain. The project received European co-financing via the Recovery and Resilience Facility under Spain’s UNICO Sectorial 2023 program, underscoring public support for digital rail modernization.

FRMCS transition and GSM-R sunset: aligning CBTC with 5G

Rail is moving from GSM‑R to 5G-based Future Railway Mobile Communication System (FRMCS), and urban rail is modernizing CBTC backbones in parallel. Validating CBTC on private 5G aligns urban transit with broader 3GPP roadmaps and Europe’s Rail priorities, paving the way for converged infrastructure that supports signalling, operations, and passenger connectivity. For asset managers, the timing is good: lifecycle refreshes, spectrum shifts, and safety upgrades are converging, making it practical to consolidate radios and fiber, and to bring more applications to the edge.

How 5G SA improves CBTC vs Wi‑Fi/LTE

Compared with Wi‑Fi or LTE, 5G Standalone adds predictable latency, deterministic QoS flows, advanced mobility, and carrier-grade security managed centrally. This improves handovers at speed, uplink reliability for train-to-wayside telemetry, and resilience in RF-hostile spaces. It also enables network slicing and edge computing to isolate critical CBTC traffic from non-critical loads, while maintaining strict performance targets. The result is a pathway to higher automation levels, capacity gains, and energy efficiency with fewer parallel networks.

Private 5G technical implications for rail and telecom

Running CBTC over private 5G reshapes the reference architecture and the engineering playbook for both railway and telecom teams.

Performance and reliability for CBTC in tunnels and harsh RF

Field validation in tunnels indicates robust radio design, with careful attention to propagation, interference, and handover strategies. Meeting CBTC latency and availability targets requires redundant cells, deterministic QoS, and precise time synchronization (e.g., PTP/IEEE 1588) across RAN and transport. Operators should expect rigorous RF planning, DAS or small-cell choices per environment, and failover engineering across dual paths and power domains.

Architecture: 5G SA core, MEC, slicing, and rail cybersecurity

Private 5G Standalone with a dedicated core enables fine-grained policy control, device whitelisting, and encrypted traffic. Multi-access edge computing (MEC) places vital CBTC functions near the track for sub-10 ms round trips, while slices or QoS flows can enforce isolation between safety-related and best-effort traffic. Cyber security must be baked in end-to-end, aligned to IEC 62443 principles, with zero-trust device onboarding, certificate management, and continuous monitoring.

Coexistence and migration with Wi‑Fi/LTE legacy systems

Most metro and suburban lines will operate mixed bearer environments for years. A migration-friendly CBTC like OPTIO can use Wi‑Fi or LTE where suitable and 5G where it adds value, easing brownfield rollout risk. Gateways should normalize telemetry and management across bearers, and test regimes must prove interoperability, regression behavior, and safety margins during phased cutovers.

Business impact and operating models for private 5G CBTC

The move to private 5G for signalling introduces new cost levers and partnership models across rail and telecom ecosystems.

Capex/opex levers and 5G infrastructure convergence

Converging signalling, operations, and passenger services onto a common 5G transport can reduce parallel networks, simplify maintenance, and enable centralized lifecycle management. Energy efficiency gains and higher on-time performance translate to measurable opex benefits. Procurement can shift from piecemeal Wi‑Fi refreshes to strategic multi-year 5G programs that bundle RAN, core, edge, and security with outcome-based SLAs.

Neutral-host models for rail private networks

Cellnex’s role as an infrastructure operator highlights a viable neutral-host model for rail corridors. Railways can retain control via private spectrum and governance while offloading deployment, operations, and upgrades to a specialist partner. This approach supports multi-tenant use cases—e.g., CBTC, operations, and MVNO passenger services—under strict isolation and SLA frameworks.

Risks, open questions, and standards for private 5G CBTC

Despite promising results, several risks and dependencies must be managed to scale safely and economically.

Safety certification (SIL-4) and deterministic performance

CBTC targets SIL-4 safety integrity levels, so the end-to-end assurance case must include the 5G transport, QoS enforcement, timing, and failover. Deterministic behavior under worst-case load, maintenance activities, and incident response must be proven and auditable. Operator playbooks need clear fallbacks if the bearer degrades, with automated reversion to redundant paths.

Spectrum strategy, lifecycle, and rail-grade 5G devices

Private spectrum availability and licensing conditions vary by country, which impacts coverage design and cost. Rail-grade 5G devices—onboard units, trackside radios, and ruggedized CPE—must support long lifecycles and secure firmware management. Vendors should align to 3GPP Releases relevant to URLLC and positioning, and ensure forward compatibility to avoid mid-life refits.

Next steps for scaling private 5G CBTC

Enterprises should turn this milestone into an actionable roadmap that balances innovation with safety and cost control.

Actions for rail infrastructure managers and metros

• Commission corridor-by-corridor assessments to map bearer requirements to performance and safety targets.
• Design a multi-bearer migration plan: Wi‑Fi/LTE where sufficient; 5G for tunnels, high-density zones, and future automation.
• Specify SLAs tied to CBTC KPIs (latency, availability, handover loss) and require MEC and QoS isolation for safety-related traffic.
• Align cybersecurity to IEC 62443 with zero-trust onboarding and continuous monitoring.
• Leverage public funding streams such as RRF-style programs to de-risk early deployments.

Actions for telecom vendors and integrators

• Provide private 5G SA with hardened cores, deterministic QoS, precision timing, and documented safety contributions.
• Offer validated reference designs for tunnels and stations, including DAS/small-cell blueprints and redundancy patterns.
• Build rail-grade device portfolios and long-term support commitments, including patching and lifecycle management.
• Prepare coexistence toolkits for phased cutovers and comprehensive test suites covering failure modes and recovery.

CAF and Cellnex have shown that CBTC over private 5G is viable beyond the lab, and the next competitive edge will come from who can scale it safely, economically, and in lockstep with rail safety cases and FRMCS trajectories.