I wouldn't do Amsterdam to Warsaw by train in one go.
I did do London to Warsaw by train last year because I found the perfect itinerary for some business I needed to conduct in a few cities.
For 80 euros I did the following
- London to Amsterdam 11am - 4pm, 3 hours in Amsterdam
- Amsterdam-Duesseldorf 7pm-9pm, 2 hours in Duesseldorf
- Duesseldorf-Potsdam 11pm-5am (seated!), 6 hours in Potsdam and Berlin
- Berlin-Frankfurt(Oder) 11am-noon, 1 hour in Słubice
- Frankfurt(Oder)-Warsaw 2pm-7pm
The scenery during daylight hours was not particularly interesting and to be honest the last leg just dragged on and on. Although on that train, you could pass the time in the dining car as recommended by Seat61. I walked through it and the food looked pretty good, but every seat was occupied the entire journey.