Wikipedia
Parafontaria
Parafontaria is a genus of "flat-backed" millipedes (order Polydesmida) consisting of 13 species native to Japan, where they are known as train millipedes. Some species exhibit periodical swarming behavior during which large numbers of individuals congregate and can sometimes slow or derail trains when they occur on tracks. Individuals vary from around as adults, and feed on leaf litter as well as soil, making them comparable to earthworms in facilitating decomposition and soil nutrient cycling.