Can I book the train tickets of Shanghai, Suzhou, Nanjing, Pingyao and Xian in advance abroad?

Dear all,

We're going to China in April. We plan to travel by(night)train from Shanghai to Suzhou, Nanjing, Pingyao, Xi'an and back to the Shanghai region. Do we need to buy tickets befor arriving in China or can we book at the arrival. This to avoid situations where for some reason the flight to China is cancelled. If we need to book in advance .. what is the period to do so?
Is it possible to cancel traintickets from abroad? How? Conditions?


Thanks