Altitude Mountain Sickness (AMS)

From my own Lonely Planet books:

There are bus journeys in Tibet where the road goes over 5000m. Acclimatising to such extreme elevations takes several weeks at least, but most travellers come up from sea level very fast - a bad move! Acute Mountain Sickness (AMS) results from a rapid ascent to altitudes above 2700 m. It usually commences within 24 to 48 hours of arriving at altitude, and symptoms include headache, nausea, fatigue and loss of appetite; in fact, very much feels like a hangover.
If you have altitude sickness, the cardinal rule is that you must NOT go higher as you are sure to get sicker and could develop one of the more severe and potentially deadly forms of disease. These are high-altitude pulmonary oedema (HAPE) and high-altitude celebral oedema (HACE). Both of these forms of alitude sickness are medical emergencies and, as there are no rescu facilities similar to those in the Nepal Himalaya here, prevention is the best policiy. AMS can be prevented by 'graded ascent'; it is recommended that once you are above 3000m you ascend to maximum of 300m daily and have an extra rest day every 1000m. You can also use medication called Diamox as a prevention or treatment for AMS, but you should discuss this first with a doctor experienced in altitude medicine. Diamox should not be taken by people with a sulphur drug allergy.

If you have altitude sickness you should rest where you are for a day or two until your symptoms resolve. You can then carry on, but ensure you follow the graded-ascent guidelines. If symptoms are getting worst, you must descend immediately before you are faced with a life-threatening situation. There is no way of predicting who will suffer from AMS, but certain factors predispose you to it: rapid ascent, carrying a heavy load, and having a seemingly minor illness such as a chest infection or diarrhoea. Make sure you drink at least 3L of noncaffeinated drinks daily to stay well hydrated. The sun is intense in altitude so take care with sun protection.

Train VS Plane:
Altitude sickness (AMS) is no joke and it is quite common to discover that the nice travellers you met on the way into Lhasa have left the next day, sick as a dog (or worse) from the change in altitude. While medicines such as Diamox can certainly help, it's best to avoid shocking your systems by rising in altitude gradually. This means of course that flying is the worst way to travel in. The train is much better, as you only start to rise above 2000m on the second-last day, giving your body a bit of chance to acclimatise before you arrive in Lhasa. Having both flown down and taken the train in, we can say the difference in how we feel upon arrival is night and day in favour of the train.