How did these operate? I understand that stations would have a tall pole fitted with a crosswise hook, for a mailbag, with a suitable loop, to hang on. But how did the station crew get the bags up or down? And how did they get to or from the trains? Apparently the trains had levers or other fittings to pick up or deliver mailbags, getting them to or from the hooks at the stations–but how did they work?
My memory of the mail cranes is that the postal worker inside the train simply swung out a pivoting hook and snagged by hand the large loop at the top of the bag with something resembling a shepherd’s crook. They did not need to be that high compared to the station because the station platform was nearly the correct height to match the passing RR cars. It was simply a matter of lifting the bag over your head and placing the top of the loop over the hook. When there was no station, they just used a stool or ladder to get the bag up, as shown in the photo on the very bottom of this link:
Summer Resort Era In the photo, you can just make out the circular ring of the pick-up strap at the top of the mail crane.
I have never heard of an automated hook on the trains (although there may have been some). I do think that they mounted the “shepherd’s crook” directly on the mail car (so that they would not rip the arms out of the postal worker), but I’m pretty sure that the arm was still swung out and retrieved manually.
Mail going from the train to the town did not need to go on the hook–they just tossed the heavy leather bag onto the platform.
Thanx 
Of course you had to pack fragile items carefully.
And avoid sending them air mail.
Little known fact: these were discontinued due to the recurring need to disentangle cartoon characters from these devices after they were unexpectedly yanked out of trains.
Check out the British documentary Night Mail from the 1930s. It’s only about 20 minutes long, but quite interesting in how the mail system (processing, sorting, etc.) worked back then. This includes a sequence on how mail was picked up and dropped off from a passing train–from what I remember, it was more sophisticated then “hook/unhook”, but I can’t remember the details enough to do the process justice.
Though you won’t find it at Blockbuster, the film is available on some documentary short video collections, especially ones on the GPO (General Post Office), home of the first significant documentary movement in film history.
I think the British system used a large basket-like contraption that swings out of the train. The mail bag is hung from a hook, and a sort of 2-sided basket (i.e. no wall in “front”) catches the bag. The basket then swings back into the train. The outgoing mail was simply dropped, but maybe there was some sort of trigger mechanism to drop it in the correct place.
My knowledge of this comes from the “fully operational” HO-gauge model which I used to have. It came with little plastic “mail bags” and a trigger mechanism to install on the track - it actually worked quite well. I think it was made by Hornby.
Yeah, scr4, that does sound like what I saw in the film (especially the bit with the basket). As for the mail drop-off, the film showed how the guys on the train more-or-less knew where to release the bag so it landed OK (though there may have been a signal or light cue to help).