I live in DC and go to NY a couple times a year, for a decade+. It can be a little faster to fly, sometimes it can be a little cheaper (but that depends a lot on when you book), but AmTrak wastes less of my time. Living in DC proper, it's faster for me to get to Union Station. Once there, maybe I'll grab a coffee and then it's time to board the train. I get on, I sit down, I plug in, and I have three hours of no hassle quiet work time. Then I come into Penn Station, catch the A train downtown, and I'm at the NY office. There's way less standing in line, dealing with security, being shutdown due to taxiing, waiting in traffic, etc.

The Acela is useful if I'm trying to make a tight time window. However, I prefer the café car booths on the Regional.

I took the Acela from NYC to DC to visit a friend. It was expensive, but I wanted to try it.

The interior of the cars was old and beat up. The tray in front of me didn't work. The bathroom door was borked and there was this persistent hint of a foul smell.

The woman across the isle was bombed and clearly going through something, but before long she fell asleep and when she woke up was docile as a lamb.

I've read that the main advantage of inter-metropolis rail travel is that you enter and exit in the middle of town. This helped for NYC and DC -- I took a short subway ride to Penn Station on the NYC side, and then walked from Union Station to my friend's house on the DC side.

Rail is the most comfortable way to travel by far. If only it were cheaper and faster.

You have to add comfort to the equation.

And don't forget about TSA checks which can be stressful.