Networks of Networks in 6G: A Key Enabler for Mission-Critical Applications

The railway industry is rapidly transitioning toward full automation and enhanced safety, driven by the need to reduce greenhouse gas emissions and revitalize secondary lines. While 5G promises improved latency, reliability, and throughput, its high deployment costs and limited coverage hinder universal adoption. Emerging research advocates the transition to 6G, supported by a vision of Networks of Networks that would allow the transparent use of different access networks (e.g. satellite, IoT). This paper extends that paradigm by introducing a broader vision centred on multi-operator resource federation, hybrid classical/quantum communications, and distributed artificial intelligence. We propose a five-layer architecture that integrates these pillars by federating infrastructure across operators for seamless coverage and combining quantum-secure channels with conventional links for robust security. Additionally, our design leverages federated artificial intelligence at the edge for real-time decision-making. Finally, we outline the primary technical challenges and research directions necessary to realize resilient, adaptive, and secure railway connectivity in the 6G era.