  In public railway systems, minor disruptions can trigger cascading events
that lead to delays in the entire system. Typically, delays originate and
propagate because the equipment is blocking ways, operational units are
unavailable, or at the wrong place at the needed time. The specific
understanding of the origins and processes involved in delay-spreading is still
a challenge, even though large-scale simulations of national railway systems
are becoming available on a highly detailed scale. Without this understanding,
efficient management of delay propagation, a growing concern in some Western
countries, will remain impossible. Here, we present a systemic risk-based
approach to manage daily delay cascading on national scales. We compute the
{\em systemic impact} of every train as the maximum of all delays it could
possibly cause due to its interactions with other trains, infrastructure, and
operational units. To compute it, we design an effective impact network where
nodes are train services and links represent interactions that could cause
delays. Our results are not only consistent with highly detailed and
computationally intensive agent-based railway simulations but also allow us to
pinpoint and identify the causes of delay cascades in detail. The systemic
approach reveals structural weaknesses in railway systems whenever shared
resources are involved. We use the systemic impact to optimally allocate
additional shared resources to the system to reduce delays with minimal costs
and effort. The method offers a practical and intuitive solution for delay
management by optimizing the effective impact network through the introduction
of new cheap local train services.
