@jeder ICE3's had the quiet compartment behind the driver seat (or at the back of the train pending on the direction of travel). There is a button there where you can switch off the lights and turn down the volume of the announcements. (Also the door doesnt open unless you press the button, which helps keep out the stupid tourists.) My favourite seat in there was at the front, because you can also put your feet up on the cabinets with the route books.
@jeder depends on the train, older trains tend to be nicer, ICE 1 in particular has compartments where you can just turn off the light. ICE 4 is just bad, 3neo is a bit better iirc
1