Has anyone taken the train and ferry from London to Belfast? We'd like to do it at night. Are sleeper cars available? Any sharing of experiences or suggestions would be greatly appreciated.

There's an overnight ferry service with cabins from Liverpool to Belfast. http://www.norfolkline.com/EN/Irish_Sea_Ferries/Belfast_Liverpool/
So, train London Euston to Liverpool Lime Street, then ferry from docks.

And note by Liverpool the ferry company really means Birkenhead which is on the opposate side of the Mersey. Leave adequate time if transferring from Liverpool Lime Street Station.
Also note the changes of operator - now Stena. Used to be DFDS / Norfolk Line / Norse Merchant depending which edition of lp you have!
R

My first question would be why you want to do the journey this way? It will be both time consuming and expensive.
However assuming you really want to do it this way, crossing points as follows:
Birkenhead (Liverpool) to Belfast - 10 hours
Cairnryan (Scotland) to Larne - 1 hour then 40 mins on train to Belfast
Stranraer (Scotland) to Belfast - 1:30 hours
You could also go Holyhead (Wales) to Dublin and travel up to Belfast by bus or train.
As far as I know there are no sleeper trains from London to any of these ports.
If you don't mind flying, you could fly from London to Belfast in an hour for less than £100
Aer Lingus: Heathrow to Belfast
Easyjet: London Luton to Belfast City or both Gatwick & Stansted to Belfast International

Yep, its usually cheaper to fly. If you must travel by ferry the easiest and most direct route would be Birkenhead - Belfast but bear in mind its an 8 hour crossing because its not strictly a ferry but more of a combination of ferry and haulier truck crossing. There are private sleeper cabins available though the last time I was on it which should be booked in advance.
The Holyhead - Dublin route is shorter at 2-3 hours but then you would have to take the train to Belfast which takes 2 -2 1/2hours.
If booked far enough in advance you may get a low cost flight from £50- £80 return and if you manage to fly into Belfast City Airport its just 5-10 mins drive to Belfast city centre.

Why on earth would you bother flying to Belfast when you can do a "sail and rail"? Apart from it being a much greener option and being more friendly to the environment, think of all that hassle, getting to and from the airport (usually miles away from where you really want to be!!), having to check-in two hours in advance of your flight, only being allowed a certain size and weight of luggage (no extra handbag allowed!), no liquids above a certain amount..... to say nothing of all those "extra" costs that are added on to your supposed "bargain" fare. I could go on, and on......
For £44 you can go by train and ferry from London to Belfast with none of the above problems. Look at The Man in Seat 61 website ( http://www.seat61.com/NorthernIreland.htm ) and Mark will answer all your questions, advising on the different routes. Just remember Mark's top tip: "never travel without a good book and a corkscrew!" - sound advice, and you'd never be let near an airplane with that corkscrew!!

Agree with Gerry on the "green" aspect of not flying...................... however:
Belfast International Airport (BFS) is 30 minutes by bus from the centre of Belfast
Belfast George Best City Airport (BHD) is 10 minutes by bus from Belfast City Centre
Paying to check a bag into the hold adds around £12 per leg, depending on who you fly with and you can put all the liquids you want in your checked bag.
You will easily pick up a flight, with checked bags and all charges for under £100 (cheaper the earlier you book)
On the other hand, rail and ferry will be lower carbon and certainly more picturesque, but will take around 16 hours.
As a matter of interest what route is quoted as £44?
have fun whatever way you go :-)