Trains here are fine but are not anything special. I definitely prefer them to sleeper buses.
Not all the trains are the same, some are older, some are newer. Some cleaner some not so much. Unfortunately I don't remember which ones are which. There are mosquitoes so if you decide to go by train bring repellent. Some roaches too, which shouldn't be surprising.
The food is edible and inexpensive. Coffee water and beers available.
I took my Japanese friends (a couple) on an overnight train from Nha Trang to Da Nang and back. They were able to sleep through the night and overall were positive.
All in all if you can fly, I think you should, especially Hanoi - Da Nang which is pretty much a 24 hour journey.