I just posted to your linked topic...for lazies like me, here's what I said.

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Well, I just stumbled onto this great forum while googling amtrak codes, lol.

Anyway, I have been putting off a train trip to Michigan from Illinois in hopes that a new code would come about in a few days. Thanks for your post, I tried that code and it seems to be giving me a discounted rate, though it doesn't work with AAA or Student Advantage. Still, not a bad discount. However, here is the real sweet thing I just noticed, I started putting in random numbers around the one you posted and got several hits. Many of them were for the same % discount but I found a couple that were for a nice 25% discount. None of them worked with any AAA or SA offers, but 1/4 off is still pretty nice. Here is what I found to work

H428 - does work for 15% off
H418 - Oddly enough, 5.3125% discount (...?! )
H416 - 8.85416% discount (what's with these wacked out numbers??)

H430, H432, H420 - appears to be one but not valid for my particular itenerary

H424 - 15% discount
H423, H427 - 20% off
H421 - code can't be booked online

H426 - 25% off
H422 - 25% off

Hope this helps everyone out. I just used H422 for a cool quarter off my price tag! Hope everyone else has similar luck. I don't know all the specs of these promos since I just got lucky with some guessing, but I am going from Illinois to Michigan, passing through Chicago, if that helps anyone out with routing questions.

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There ya have it, good luck ya'll.