According to my own experience, visiting Tibet from Beijing by train is unforgetable and enthralling. Here is information in detail for those who would like to try this way.

How far?
Beijing to Lhasa: 2,540 miles/4,064km.

How long?
Beijing to Lhasa:45hr 8 min.

Departure and arrival time?
The train T27 (Beijing-Lhasa) leaves at 8:09 pm and arrives at about 4 pm the third day.

How much?
Beijing to Lhasa: About 115 UD dollars (hard sleeper)

Buffet or banquet?
Not bad as far as railway fare goes, but basic. Bring instant noodles if you're worried.

Sitting comfortably?
Yes, if you get a bottom berth. Slightly more problematic, during the day, if you're in a top or middle berth.

Make sure you pack...
A story book, a camera or a mobile that can take photos, a pairs of shoes that is easy to take on and off, snacks to share, antibacterial hand gel – and lots of loo paper.