The envelope said, "Open at Sunset"
The encoded message inside, once decrypted, read as follows:
To Polaris from Ursa Minor,
Five car train - made up in Frankfurt now in Nuremberg - contains three hundred civilians.
Names provided by NKVD. Journalists, Intellectuals, Anti-Comunists, Liberals, and families. Men, women, children, all to be liquidated on arrival in Eastern Zone.
Need extraction. Help. Situation desperate.
I didn't crack it. I didn't even know where to start. Some folks on the Unfiction forums cracked it, the explanation is halfway down this page of the thread. It was a Seriated Playfair cipher with a key of SANTI(A)GO.