Lhasa is 3700 m high. Recommendation is to ascend no more than 300 m per day. The train to Lhasa may or may not allow one to follow this recommendation, it ascends above 5000 m and descends along its route. Mild altitude sickness symptoms are to be expected at altitudes above 3000 m. More severe symptoms, marked by ataxia and worsening of mild symptoms, necessitate immediate descent. Test for ataxia by walking heal to toe (like field sobriety test). Diamox, used to reduce the symptoms and aid adaptation is contraindicated for me.

References online

CDC Health Information for International Travel 2008
Chapter 6 Non-Infectious Risks During Travel
http://wwwn.cdc.gov/travel/yellowBookCh6-AltitudeIllness.aspx

Outdoor Action Guide to High Altitude: Acclimatization and Illnesses
by Rick Curtis, Director, Outdoor Action Program
http://www.princeton.edu/~oa/safety/altitude.html

Acclimatization and altitude sickness
This info is taken from "Trekking in the Everest Region", 3rd edition.
http://www.project-himalaya.com/info-ams.html

Lhasa train elevation profile
High Tea : Railway to Lhasa
http://vistet.wordpress.com/2007/05/14/high-tea-railway-to-lhasa/