Yes, you can travel by train to Danang station then travel on land to Hoi An

Hoi An is about 30km south of Danang, and it features on many visitor‘s itineraries.  There is no railway station at Hoi An, but there are regular buses, minibuses & taxis from Danang to Hoi An, taking between 45 minutes and an hour.  The bus fare is about US$3, a taxi will cost in the region of US$9-15 depending on your negotiation skills.

Air-conditioned soft sleeper is the recommended choice for most western travellers, for any journey involving overnight travel.  Air-con soft sleeper cars have western-style toilets at the end of the corridor. 

However, air-con hard sleeper is perfectly acceptable if you‘re on a budget or if all the soft sleepers are sold out, so don‘t rule it out.  Some hard sleepers have squat-type toilets only, if that‘s an issue for you.  Air-con soft seat is a good choice for daytime journeys, but not for overnight trips as you can‘t sleep properly in a seat.

Quoted from www.seat61.com

Trains SE1 to SE8...

The best trains are the ones with ‘SE‘ train numbers, equipped with modern air-conditioned coaches. 

They have:
soft class air-conditioned sleepers (4-berths per compartment).
hard class air-conditioned sleepers (6-berths per compartment)
air-conditioned soft class reclining seats.
a restaurant or buffet car.
in soft sleeper, simple tray meals are served in your compartment along with mineral water, included in the fare.
Train SE1/SE2 also has private 4-berth air-conditioned sleepers of a higher standard attached between Hanoi, Hue & Danang, see www.livitrans.com.

The coaches on these ‘SE‘ trains have large picture windows, unobstructed by the wire mesh that protects other trains‘ windows from stones.  Choose these trains if you can. 

Quoted from www.seat61.com

Privacy-loving westerners often ask this.  The answer is that in theory yes you can, but it can‘t be guaranteed that the train staff won‘t allocate additional passengers to the berths they know to be empty. 

My advice is don‘t bother, just book 2 beds in a 4-berth soft class sleeper.  The Vietnamese may view it as selfish to take up 4 beds when you only need 2, given that places on their national transport system can be in short supply at times.  You‘ll be safe and comfortable sharing a 4-berth soft sleeper, and might actually meet some real Vietnamese people this way!

Quoted from www.seat61.com

Yes, yes, and no...  All trains require a reservation, so you need a specific ticket/reservation for each individual train journey you make.  If you want to travel from Saigon to Hanoi (or vice versa) stopping off on the way, this is not a problem, but you will need to book it as a series of separate journeys, with a separate ticket for each leg either bought in advance or bought as you go along.  You cannot buy an open ticket and hop on and off trains without a reservation.

Do I need a reservation?  Can I stop off along the way?  Can hop on and off?

Yes, yes, and no...  All trains require a reservation, so you need a specific ticket/reservation for each individual train journey you make.  If you want to travel from Saigon to Hanoi (or vice versa) stopping off on the way, this is not a problem, but you will need to book it as a series of separate journeys, with a separate ticket for each leg either bought in advance or bought as you go along.  You cannot buy an open ticket and hop on and off trains without a reservation.

 Quoted from www.seat61.com