How might someone have gotten a note forwarded from a transport train headed to Auschwitz?

Question

A relative who is no longer living told me that when her uncle (Jewish, living in hiding in France) was discovered and put on a train transport to Auschwitz, not long before liberation, he threw a note to his sister out of the train to let her know his fate, and this note was miraculously delivered to his sister in London.

How might this note have reached its destination?

I'm trying to picture this, step by step. He would likely have been in a windowless cattle car -- so how did he get a note out of the train? Whom might he have given it to? Would this have been from the station where he was first put on the train, or farther along the route? Would the note then have been sent through the postal service, or by some other means?

Edit: I imagine the uncle and his wife would have been killed upon arriving, or shortly after arriving, in Auschwitz. The officially-encouraged-postcard hypothesis seems unlikely to me, partly from the way the story was told to me, and partly because my impression is that towards the end of the war, Germany was hanging by a thread and would not have bothered with this type of PR any more.

The way this family history was explained to me, it was a note thrown out of the train, and it was considered remarkable that the note reached his sister.

Answer 1

To expand on Jeff's answer, many of the journeys would take several weeks, moving from one transit camp to the next.

Prisoners were given post cards with idyllic scenes to write to family and friends to indicate how well they were treated and extoll the virtues of their new homelands in the east.

And indeed it was always possible (though risky) to try and write a note and let it slip through the wooden planks making up the side of the rail cars. But such notes would be unlikely to be found and sent on. Not just would the chances of them being found be small, censorship and plain old fear would prevent most people from trying to send them on if they did find them (and how to know where to send them? Or even have the means to if you did know?).

A note slipped from a train in France or Germany on the way to Poland would have to travel through Vichy France and fascist Spain to Portugal before getting to Britain. While possible in theory, in practice it'd be highly unlikely to ever arrive unless the person initially finding it was involved in the secret networks set up by the OSS and other organisations to help downed allied pilots get to England (and many of those were caught and sent to POW camps).

Far more likely is that someone after the war invented the story to have something interesting to tell. Not dissimilar to hundreds of thousands of Dutch and French (and no doubt others) suddenly having been part of armed resistance groups after the war, when during the war those groups numbered a mere few thousand in either country (of course more in France, as it's bigger, but in both cases a tiny percentage of the population).

Answer 2

In at least some camps, inmates were encouraged to write, although certainly heavily censored, postcards/letters to family to allay suspicions of both families and international agencies like The Red Cross. Part of the motive also may have been so money and food was sent to inmates which were probably stolen.