This train is a JNR Class D51 steam locomotive. The D51 was used in Japan from 1936 to 1951. A total of 1 115 were built, the largest number of locomotives in a single class in Japan.
Another titbit: there is still one operating D51 at the Yuzhno-Sakhalinsk railway station in Russia. It was left behind by retreating Japanese forces at the end of World War II.
The train in the photos are parked at Asukayama Park in Ōji. Isn't it a beauty?