Hi, there are no direct trains between the two places. However, one direct train-K335 is available from Chongqing to Xiamen. Thus, you can take a coach to Chongqing first, and then take the train to Xiamen.

Taking trains from Deyang to Xiamen is annoying since it will cost more than 40 hours and you need to transfer. I suggest you take flight from Chengdu to Xiamen and the flight only needs 2.5 hours. You can book flights here: https://www.travelchinaguide.com/china-flights/