What's the fastest Xian to Chengdu rail (and does it make it faster if I change trains)?

The fastest one is T7(06:42/20:40) from Xian to Chengdu, which runs about 14 hours. It seems that there is no need for you to transit in another city if you want to get to Chengdu earlier. For example, although there is high speed G-type available from Chongqing to Chengdu, if you transfer in Chongqing, it still takes you about 10 + 2 hours, saving 2 hours at most. But it is really not convenient. You can check the detailed schedule and duration on the website.