I wish people would not post incorrect information on here as though it is fact.
First, the D trains(bullet) take just over 6 hours, not 4 hours, from Bejing to Changchun, or Changchun to Beijing, and run during the daytime.You sit in aircraft type seats and limited refreshments come round on a trolly.Flights between the two cities are quite heavily booked, and can be comparitively expensive, especially at holiday time.
There are several night sleepers that run between Beijing and Changchun.They all conveniently depart in the late evening, and arrive between just after 6am and up to 8am. You can get a 'soft bed'(1st Class) which costs around 400RMB and is quite luxurious with 4 in a compartment, individual LCD TVs at the foot of each bed, newly sterilised stereo headphones for each passenger, and of course slippers. The train is clean and comfortable, so you can get a good night's sleep, save a hotel bill and avoid those long treks out to airports.
On the train, car 10 is the restaurant car and bar. Good food is available,and if you are a nightbird you can watch the big LCD tv in the bar.
Foreigners can buy train tickets quickly and easily at special counters in Beijing and Shanghai manned by multi lingual staff.In Changchun there is a special counter upstairs in the booking office for 'soft bed'tickets. Of course, any hotel can get the tickets for you, for a very modest few RMB add-on. My last was 5RMB extra per ticket.
i would not bother with a flight from Harbin to Beijing either.