The accident just before midnight Monday in Yokosuka caused three passenger cars to derail inside a tunnel, The Yomiuri Shimbun reported.
About 700 passengers were aboard the limited express train when it hit the landslide, believed to have been caused by torrential rain. Five passengers were seriously injured, while five others and the driver were slightly hurt.
An area of soil about 40 feet wide and 50 feet high fell on the tracks about 65 feet before the entrance to a tunnel. Trees and fences meant to stop falling rocks also fell on the tracks, said police and officials with Keikyu, the operators of the line.
Investigators said the train was traveling about 46 mph when the driver saw the blockage about 100 to 130 feet ahead. The driver hit the emergency brake, but the train travelled about 230 feet before stopping. Four cars of the train ended up inside the tunnel, where three of them derailed.
At about the time of the accident, a weather observatory issued a "record-setting" torrential rain warning for the Yokosuka area.
Service on that part of the line was still suspended Tuesday afternoon.