What was the 'sealed train' Lenin used to return to Russia from Germany in 1917?

It was a diplomatically-sealed train originating in Switzerland and bound for Russia, traversing Germany and Sweden. The Germans allowed the transit, not taking any chances that Lenin would disembark in Germany to spread his Communist propaganda, but fully welcoming the idea of him inciting revolt in Russia, which was at war with Germany.

For this, Lenin's foes in Russia had denounced him as a "German spy". While he most likely was not anybody's spy, the revolution he eventually led certainly played into Germans' hands.