By Rail
In a peculiar public-private partnership, Vietnam Railways has done deals with various private companies to attach their carriages to the trains that run between Hanoi and Lao Cai station - 38 kilometres from Sapa.
There are at least five companies - some with bizarre names like Livitrans, Ratraco and Tulico offering this service.
I've tried most and was happy with all but my most recent experience on Tulico. On that trip in October 2013 the carriage was poorly maintained, there was only a dirty squat toilet and there was a nasty banging sound coming from beneath our cabin that made sleep near-impossible.
Other services offer a reasonable level of comfort in four berth soft sleeper cabins at around $30 - $40 one way. Prices have risen significantly in recent years with no discernible improvement in quality.
Tickets for these should be purchased via your hotel or a travel agency. I could not find any ticket outlet at Hanoi Railway Station selling these privately operated carriages.
The most comfortable and expensive carriages in service are operated by Victoria Express which is run in conjunction with the Victoria Hotel in Sapa. The carriages are decorated in colonial style and there is a dining car. The dining car is of limited value given that services operate through the night and travellers are likely to have had dinner prior to boarding.
I have heard many recent reports that the price premium is too high for the negligible improvement in quality.
By Road
If you're planning more extensive travel through Vietnam's north, road travel is increasingly straightforward with big improvements in roads heading west from Sapa and into Laos and east towards Ha Giang, Cao Bang and Lang Son.
There are buses operating these routes and you can also incorporate them into a private tour or simply book a private car in each place for point to point travel.