Journey planner for finding intercity train connections in China?
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Accepted answer
In the end, I found two ways to do this, but both require knowing at least a bit of Chinese.
The clunky but straightforward way is to use the ctrip.com website in Chinese, select 中转 from the far-left dropdown, and manually input the connecting city. (This feature is not available on the English version of the site.) Here's Beijing to Xining via Xi'an (only), with Beijing-Xi'an trains on the left and Xi'an-Xining trains on the right:
The slick but more labor-intensive way is to install the Chinese Ctrip app aka 携程旅行, and then just do a regular train search, which will automatically show suitable connections through any city. Here's Beijing to Xining again, which shows those connections through Xi'an: