[FROM: Matt Sho]
From everything I've seen (currently I'm lvl 88 or so)...
A passenger car with 215 ppl will earn more than a mail car with 2300 mail (the "genesis" set pass and mail cars).
Typically a good/fast conversion is "10 mail = 1 pass" for when comparing the different cars (though I think if you're looking for a bit more accuracy, you could probably go "15 mail = 1 pass").
As others have noted, you have a limit on how many waiting passengers you can have, but there seems to be no limit on how much mail can be built up.
I've found that I almost always prefer passengers over mail (by level 80, the mail carts max-out at about 1800 letters whereas passenger cars carry 200 ppl, and it stays that way until lvl 88 when you finally get a larger mail car).
There is ONE very noteable mail car: the "express mailer II" (7000 letters). You can buy 3 from the shop, and you can get a few others as either achievement awards or level-up awards. Easily I'd say its like having 2 or 3 regular mail cars, so its obviously a mail car you always want in use.
If you run 3 trains for money, I suggest 2 as "all passengers" and 1 for "all mail", so that you almost never run into a situation where you have to sent out empty pass cars or have to swap a few pass for mail. If you run 2 trains, maybe do 1 as all-pass and 1 as half-and-half (or maybe 2/3rd's pass cars and 1/3rd mail).
>>> FYI, using this setup, I'm able to run a full all-pass train on one of my international tracks to NPC's
You can also (every few days) check the "specials" and "vintage" offers in the shop (just left of the engines tab in the shop... not sure if its available in the facebook-version of the game, but if you're smart you'll login daily to the actual Train Station portal (www.trainstationgame.com/portal)
and play from there, since there is a "mini-game" (random daily prize, usually material but sometimes its a few gems) and 10% bonus (when buying gems).
...... anyways, I've seen BIG stuff there, like 145% material cars (which you can't usually buy until like level 140+) and passenger cars (ranging from 215 +15% to 235 +20% ... even the odd 250 +65% maybe once every 3 months).
Thanks to the special and vintage offers in the shop, nearly all my material cars in use are of the 145% size. I mean why spend big-bucks to buy gems to spend gems on a +60%, +80%, or +100% car, or wait ages to be level 130 to have those sizes become available for coins, when you can buy a +145% in the specials/vintages.
And YES, I have EMPTIED my materials to buy up the max 8 cars of an 8-car buy-limit of a +145% wood cars (or whatever material it carries). (if you're not familiar, click "materials" on upper-right corner of screen to see what you have stockpiled up. You can always sell off stuff. Like I've got 6 of the +145% silicon cars going, and every few days I sell all my silicon off in order to buy some special/vintage.