How do I book a train online? Where is the form?

Well, you need to fill in your departure and arrival city name respectively in the train search form first. After searched out the train No., you can select your preferred train and submit an online booking directly.

"you need to fill in your departure and arrival city name respectively in the train search form first"
Where is this train search form please?

There was an item on the news yesterday about more tickets being available online (CRI English in Beijing). Do you know where they are available from? Thank you. In Wangjing there is a small ticket office just off Huajiadi. Can I buy long distance (Beijing-Xi'an) tickets there? My Chinese is not very good. Thank you.

You could search the train on the page below first and then submit an online booking enquiry: https://www.travelchinaguide.com/china-trains/ And of course, you can also buy the tickets from Beijing to Xian at any train tickets offices in Beijing. Sorry for that I haven’t heard any news about more tickets being available online. And in my option, train tickets have been and will continue being under great demand in China.