After reading a post here somewhere about how there's a 'TGV Poste' service (the fastest land mail carrier in the world), I noticed that Loco's TGV and Electra-Star trains lack this ability. So I quickly just whipped up ones for them. They use the same graphics, as I'm no artist by any stretch of the imagination.
Anywho, just extract them into your \ObjData folder. To get them in the game, you'll have to go into the scenario editor and enable them.
Update (Oct 22): I updated the subject line to conform with the guidelines for this section. I also updated the file to include an APT mail carriage. This should help prevent people from stringing a bunch of driving carriages together.

The trains are configured for the extended (x2) reliability mod.
You'll have to go over to Steve's Forum to get it, as these ones are being b*tchy and not letting me update the file.
http://forum.locomotiondepot.net/download.php?id=14
"As soon as men decide that all means are permitted to fight an evil, then their good becomes indistinguishable from the evil that they set out to destroy."
- Christopher Dawson, The Judgment of Nations, 1942