Why Are Restrictions Imposed When It Rains? When you’re driving and get caught in a heavy downpour, your forward visibility is significantly impaired. However, in the case of railways equipped with signal safety systems, it is rare for poor visibility to render operations impossible. The effects of rain are primarily felt in civil engineering structures and equipment. In other words, when heavy rain falls and large amounts of water pour down, there is a risk that civil engineering structures and equipment could be damaged or blocked, making it impossible to operate trains safely. There have also been actual cases where trains carrying passengers were directly struck by mudslides. To avoid such situations, operators monitor rainfall conditions and consult weather forecasts to implement restrictions and suspend train operations in order to prevent accidents before they occur. In recent years, in particular, there has been an increase in cases where preventive service suspensions are implemented early on when heavy rain is forecast. First, as a specific example of damage, we will examine the damage sustained on the Tadami Line between Aizukawaguchi and Tadami during the heavy rains in Niigata and Fukushima in July 2011.
The Relationship Between Civil Engineering Structures and Rain When we talk about civil engineering structures for railways, in addition to constructing roadbeds on the ground, there are also “embankments,” “cuttings,” “viaducts,” “bridges,” and “tunnels.” You might wonder what an “embankment” is—in short, it is a mound of earth. When laying tracks above ground level, soil is piled up to create an embankment-like structure, which serves as the foundation supporting the tracks. After heavy rains, news reports sometimes state, “Track bed erosion has occurred on the XX Line.” This is easy to understand if you pile up soil and pour water on it: when soil absorbs a lot of moisture, it becomes loose. If the track bed supporting the tracks becomes loose and collapses due to rain, it means the foundation supporting the tracks has failed, so trains cannot operate. This is true not only for conventional lines but also for the Tokaido Shinkansen, which was built earlier; extensive use of embankments was common during its construction. This was done to create grade-separated crossings with existing roads and other infrastructure, thereby avoiding the need for railroad crossings. However, in subsequent Shinkansen projects, concrete viaducts have become the standard, as they are more resistant to rain. This is the reason why the Tokaido Shinkansen cannot implement snow-melting water spraying like the Joetsu Shinkansen does. Regardless of whether the water is natural or man-made, if a large amount of water pours onto an embankment, there is a risk that it will become loose. Therefore, it is not possible to spray enough water to completely melt the accumulated snow. That said, it is not feasible to completely rebuild all the civil engineering structures. Consequently, on the Tokaido Shinkansen, improvements have been implemented, such as installing drainage pipes inside the embankments to prevent water from pooling and applying concrete lining to the slopes (the surface of the embankment) to prevent erosion. Not only on embankments, but also in cuttings, along tracks adjacent to slopes, or near tunnel portals, soil and sediment loosened by rainfall can flow onto the tracks. Even if a full-scale collapse does not occur, rockfalls on slopes can occur, and falling rocks can block the tracks and cause disruptions. In addition, rain can cause the ground to soften, leading to fallen trees that may collapse onto the tracks and cause disruptions.
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