Flying takes 4 or 5 hours out of your sightseeing day in getting to a remote airport, checking in, taking the flight itself, collecting your bags and getting back into the city centre. Hoi An has no station, but it's just 30km by bus or taxi from Danang.
These have 4-berth air-con soft sleepers of the same sort as the best refurbished regular Vietnamese Railways ones, but with upgraded on-board service and (of course) significantly higher fares, see the photos, information & advice below.
Trains SE5 & SE6 were allegedly re-equipped with some of the latest modern seats cars & sleeping-cars in January 2018.
SE7, SE8: Only runs at busy times, ask an agency if it's running a month or two ahead SE19, SE20: Air-conditioned soft sleepers, air-conditioned hard sleepers, air-conditioned soft seats, air-conditioned hard seats, ordinary hard seats.
Rice fields, palm trees, water buffalo, Vietnamese towns and villages...
You get a real insight into Vietnam when you travel by train, both urban and rural, which you don't get from 35,000 feet.