Go Tibet on 18 May 2013, when can we apply for Tibet Entry Permit & this permit valid for how long?

Going there in May2013, when can buy train ticket for Chengdu to Lhasa? buy online? any requirement?

Hi, as long as you get the Entry Permit, you can buy the train ticket 10 days ahead.
For convenience, you can buy the ticket online. I suggest you mail to travelchinaguide.com|booking and the operators will tell you how to get the train ticket.
You need apply for the Entry Permit in advance, about a month ahead for secure. Also the permit have to be applied by a travel agency, not a personal. The detailed information, you can get in the website:
https://www.travelchinaguide.com/faq/tibet-travel-permit/