My recollection of train #67 last summer is that the (presumably Amfleet II) coach always had at least some overhead lighting on. I didn't think it got terribly dark at all, but I may not be a great judge of that, because my vision seems to not have significant difficulty in low light.
I'm not sure I got out of my seat during the night on that trip, but I certainly don't think I would have used the 2 AA maglite that I pretty much always carry if I had.
#67 is a Northeast Corridor train, and since most trips on the Northeast Corridor are short, daytime trips, it is possible that the crew there does not dim the lights as much as is done on the long distance routes; I don't have firsthand experience with the long distance routes to comment on that.
I believe there's also a coach attendant on the long distance trains but not the Northeast Corridor trains. And I'm not sure how easily you'd get a coach attendant's attention late at night without disturbing other passengers and without leaving your seat; I haven't seen airline-style call buttons on Amtrak.
