The armoured train Śmiały, sometimes PP 53 and officially Armoured Train number 53 was an armoured train of the Polish Army that saw significant action during the German Invasion of Poland in September 1939. The train in the end served under four flags—Austrian, Polish, Soviet, German—and fought in several wars from 1914 to 1945. Śmiały distinguished itself in the Battle of Mokra, after which it withdrew eastwards, taking part in the Battle of Brześć Litewski. After the Soviet invasion of Poland on September 17, the train left the western front via Kowel to Lwów, where it fought in the Battle of Lwów. On September 22, 1939, abandoned by its crew, it was seized by the Red Army.

The Battle of Mokra took place on 1 September 1939 near the village of Mokra, 5 km north of Kłobuck and 23 km north-west of Częstochowa, Poland. It was one of the first battles of the Invasion of Poland, during the Second World War, and was one of the few Polish victories of that campaign and the first German defeat of the conflict.