Maintaining reliability is a key challenge for electric grids as they endure more frequent extreme weather and utilize larger amounts of variable renewable energy in response to the climate threat. With traditional reliability approaches becoming less viable, alternatives are being sought. We examine using the U.S. rail system as a kind of nationwide backup grid, in which containerized batteries, or rail-based mobile energy storage (RMES), are shared among regions to meet demand peaks, improve resilience, and relieve transmission congestion. Such an approach could also accelerate decarbonization of the rail sector. We find that RMES is a feasible reliability solution and quantify its cost-effectiveness relative to reliability-driven investments in stationary storage and transmission infrastructure. While no technical barriers exclude mobile storage from electricity markets, addressing interconnection and cost challenges and revising regulatory frameworks will be needed to deploy it at scale.