Should I purchase Lijiang train tickets in advance?

My wife and I shall stay in August about at least 7 days in Kunming, Dali and Lijiang. Then back to Kunming to go to Chengdu. Do we have to buy train tickets already on arrival or wait at least 48 hours before departure.

Also can we get tickets from chengdu to another place in Kunming ?
Your help will be much appreciated.