A sealed train is one that travels internationally under customs and/or immigration seal, without its contents legally recognized as entering or leaving the nations traversed between the beginning and end of the journey or subject to any otherwise applicable taxes. The practice was used a number of times throughout the 20th century to allow the migration or transport of controversial individuals or peoples.
The most notable use of a "sealed train" was the return of the Vladimir Lenin and other Communists to Russia from exile in Switzerland in 1917, during the First World War. This was allowed by Germany with the aim of increasing unrest in Russia and to cause Russia to leave the war, which also occurred.
That the train was "sealed" has been stated to be a propaganda ruse in order to give the false impression that Lenin and Germany did not work together. Actually, Lenin got off on several occasions, and stayed overnight in a hotel in Germany. Recently discovered evidence from Russian archives has been argued to add further support to the accusation that Germany supported Lenin financially.
- Was Lenin a German Agent? https://www.nytimes.com/2017/06/19/opinion/was-lenin-a-german-agent.html