Well, I tried to post this while I was on the train the other day but had no luck. Ironically, it seemed that the wifi wanted to work for just about everything but railroad-related websites...
There's now wifi on, at a bare minimum, the Silver Star...though I think it's probably live on the whole single-level system (or close to it now). And yes, it works, though the speeds are often somewhat reminiscent of the 1990s (which sadly brings to mind an alt-hist where Amtrak had wifi starting then...yes, I know the limits of the technology). A text-based MOO (basically a MUD) I ran operated smoothly as did most messenger applications, but AU would not load (and a lot of other sites were painfully slow). There's wifi but it isn't perfect...but it's a lot better than nothing.
When I saw the card in my room on Monday night (3/14) announcing this (I was boarding at ORL) I did a mildly embarrassing fist pump (you have no idea how happy I was to see this) given how long there's been a teasing AmtrakConnect wifi network visible-but-not-working. The card listed all of the eastern overnight trains (Auto Train, Cardinal, Meteor, Star, Crescent, and LSL); the Palmetto already had wifi as a day train (it was in the same loose boat as the Adirondack and Pennsylvanian IIRC) and so didn't get a note, though I think wifi is out in all the cars on all of those trains now as well.
The slow connection is better than nothing (and understandable given a lot of the areas the trains go through) but nothing would have been better than the "teasing". I suspect this was one of the first trains with it; the SB Meteor on 3/10 was still "teasing" so the system probably got activated over the weekend. Either that or I was just on...basically the first run of the service (something that would truly thrill me in some ways) and it'll be going out elsewhere over the next few weeks.
Of particular note, this basically kills off the last (theoretical) advantages the Regionals had over the Silvers for me: Now the Silvers have arguably better seats, the ability to book a room, and (with the Meteor) far better food service.