Zag gave you good advice. Remember that most interrailers have no money to spare on expensive city center hotels, those next to the station. It's common standard to sleep in the night train, anywhere. The six seats in a cabin turned to a three bed "sofa" are rather short, about 1.7 m as far as I remember. And both of the headrests of the joining seats can't be leveled fully, so you sleep with the feet and the head upwards.
So sleeping on the floor in the corridor, between the carriages or -with the older trains- in the luggage compartment above the seats if you're slim is what you've to expect. But wine and beer is cheap on the continent, the lulling rattle of the train and a bottle will get you asleep. But stay away from the toilet when unfolding your sleeping bag.