Overnight trains are fun, but this one comes with the added bonus of a brilliant view between Da Nang and Hue (on the right hand side of the train going north).
We paid 450,000 dong (about $30) each for a top bunk berth in a four-bed carriage. Lower bunks are more expensive, but probably nicer for views and storage space.
The train comes all the way from Ho Chi Minh City to Hanoi. If you are getting on half way then you may need to strip the bed of its old linen and ask the not very helpful guards for 2 new sheets.
Food is included, but not particularly nice (cabbage and mackerel stew on boiled rice anyone?). Vendors follow the official trolley selling much nicer fare.
There are several trains a day. We caught the 14h15 from Da Nang so that we'd get the coastal views up to Hue. Everyone seemed to bunk down and sleep by about 20h30 so arriving in Hanoi at 04h30 did not seem too traumatic! And then you are just in time to dump your luggage and join the early morning crowds at Hoan Kiem lake.
Buy tickets in advance from the train station (not sure if you can buy from a station other than your departure one- tried to find out and failed). The earlier you buy the better as tickets do sell out.
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Plus I was ill, so I got very little sleep and walking around Hanoi like that the next day was hard.
The food looked ok early on, but the same fried eggs were being served in the morning as the night before. I recommend taking something of your own, and there is an amazing "supermarket" in DaNang to stock up on Vietnamese snacks.
You will find very little western fare, and it's really interesting to walk around. The market is also good, but be more wary of eating these snacks on the train. Some are not sealed, and it is the last place in the world you want your stomach to be sick.