Radio Access Network for 4LM

Why It Matters

A robust RAN is critical for CBTC reliability; any communication lapse triggers emergency braking, so the 4LM radio design directly impacts service capacity, safety and the scalability of digital signalling across global metro systems.

Key Takeaways

• 4LM covers 150 km, 200+ trains, 100+ stations
• Seltrac CBTC uses radio‑based positioning, not track loops
• ~1,450 directional aerials installed at ~200 m intervals
• Continuous big‑data monitoring validates RF performance and packet loss
• Redundant AP coverage prevents service impact if one fails

Pulse Analysis

London’s Four Lines Modernisation marks a watershed moment for urban rail, swapping legacy track‑loop signalling for a fully radio‑based CBTC architecture. By leveraging Hitachi’s Seltrac platform, the project introduces Automatic Train Operation, Supervision and Protection across 40 % of the Underground network. The shift to radio positioning not only frees infrastructure from costly track circuits but also enables moving‑block operation, which can increase line capacity by up to 30 % while maintaining strict safety margins.

Designing the RAN presented unique engineering hurdles. Engineers opted for tunnel‑mounted directional aerials rather than radiating cable to minimise interference, spacing each unit about 200 metres apart—resulting in roughly 1,450 access points. Heritage platform constraints forced installations at platform edges, and OFCOM power‑output limits dictated careful RF planning to avoid spectrum congestion. Validation involved static and dynamic testing, measuring packet loss, latency and consecutive communication gaps to certify that the system meets the stringent reliability thresholds required for automated train control.

Once operational, the network is kept in check by a big‑data analytics platform that ingests billions of telemetry points daily. Continuous monitoring of RF levels, packet delivery and equipment health allows rapid detection of degradation and automated re‑tuning of both wayside and onboard units. This data‑driven approach not only safeguards the 70 Gbit of daily traffic but also provides a scalable model for other metros seeking to modernise with digital signalling, underscoring the strategic importance of resilient radio communications in next‑generation rail systems.