We have done this journey quite a few times now - the beds are ok and come with linen - pillow, duvet and sheet(we all managed to sleep but it's not dunlopillo by any stretch of the imagination). You will also get a bottle of water and there is a trolley selling various items (tea, crisps etc) I have never tried the food which has looked everything from pretty good to scary. Take some snacks (there's a hot water tank in each coach so pot noodles are easy to do), loo roll and easy slip on shoes for the toilet in the night. On the return journey the conductor may try to fob you off with used bed linen, be polite but insistent and he will change it eventually - or swap with unused from another cabin which is what we have done on a few occassions - going out it's not an issue as the train starts from Hanoi.

I have no experience with the Hue train but on the Sapa train, 'hard sleeper' means you're six to a cabin instead of four and the mattresses are thinner.