(2) According to the distribution map of railway trunk lines in China, the best travel route from Yanzhou to Lhasa is: Beijing-Shanghai line → Longhai line → Lanqing line (non-railway trunk lines) → Qinghai-Tibet railway;

(3) The Qinghai-Tibet Plateau has an average altitude of more than 4,000 meters, and the climate is cold. You should take a cotton-padded coat to keep out the cold before you leave;

(4) Farmland, towns and cities in Qinghai-Tibet region are mainly distributed in valley areas with low altitude;

(5) Due to global warming, glaciers are melting faster, the snow on the mountains is decreasing every year, and glaciers are retreating.

So the answer is: (1) After the Qinghai-Tibet Railway is completed and opened to traffic, it is convenient for tourists to enter and leave Tibet;

(2) Beijing-Shanghai Line → Longhai Line → Lanqing Line → Qinghai-Tibet Railway;

(3) The Qinghai-Tibet Plateau has an average elevation of more than 4,000 meters and a cold climate;

(4) Most of them are distributed in valley areas with low altitude;

(5) Global warming will accelerate the melting of glaciers.