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 Bolsheviks to Russia from their exile in Switzerland in 1917, during the First World War. This was allowed by Germany with the aim of increasing unrest in Russia and hopefully to cause Russia to leave the war, which also occurred.
That the train was "sealed" was not entirely true as Lenin got off on several occasions, and stayed overnight in a hotel in Germany. Recently discovered evidence from the Russian archives has been said to add further support to the accusation that Germany supported Lenin financially, although this was doubtless the costs of the said journey, etc.
- Was Lenin a German Agent? https://www.nytimes.com/2017/06/19/opinion/was-lenin-a-german-agent.html