I think you'd find the best Christmas decoration views on the NEC between NYP and BOS, particularly in Connecticut. Lots of houses, lots of money, and charming New England towns, especially along the shore line should equal great decorations viewable from the train. The trick would be to get the conductor to dim the lights.
I'd suggest Acela first class, as the car attendant has done it a couple times while I was in FC (using an upgrade) on non-holiday season evenings. Acela has the advantage of really big windows. For best views, choose an aisle seat, not a window seat or single seat, to see equally well out both sides.
Ideally, having a roomette and turning out the lights works the best. I've watched 4th of July fireworks from my roomette a number of times on the Lakeshore Ltd tr #48 going through numerous small towns in Indiana. Once upon a time in the early '80s, I watched Indiana fireworks from the rear platform of a private car (LV 353) at the back of the Broadway Ltd on PRR tracks. The fireflies were thick that night as well. Watching fireworks from airplanes at 30,000 feet appear to be about an inch or so in diameter. I've done that a number of times as well back when I was regularly flying on business.