Hard to be sure what it is, easier to be sure what it is not. Not Randall, not Lee Lines, not DeHanes, not Jerry Brown, not John Daniel, obviously not Richart or Art Varney, and definitely not JAD. None of them made any such thing in standard gauge. The entire JAD standard gauge line consists of the M10000, the Hiawatha, and the Rail Chief sets. I'm with Rob, nobody has made Amtrack coaches in standard gauge commercially, it has to be a one-off home-made.
It would be good to hear from Jim Waterman on this, because the closest match I can think of is that the extruded aluminum sides do look like they could possibly be Lee Lines that somebody has done some customizing with. The Lee Lines extrusions could be cut any length, so 30" car is not out of the question. But if it is Lee Lines parts, it has been extensively homemade/hacked/kitbashed. Jim would be able to tell at a glance if they are his extrusions... I think they are too small and don't match the Lee Line extrusion, but Jim would know better.
Also, AristoCraft did some nice long Amtrak cars in aluminum for G Scale, someone may have used parts from these.
david