is there a train from Zhangjiajie to Sanjiang, Guangxi near Longsheng

No, there is no direct train between the two places. Then I have a suggestion for you:
take a bus or train from Zhangjiajie to Changsha. At Changsha West Bus Station, there is a direct bus to SJ, leaving at 09:20.

I found tha tfrom Zhangjiajie to SJ.There are 3 trains 1) #2011: 17:46/01:08 2) K1473: 18:30/01:35 and 3)#1808/1805: 07:20/17:50
11 - Zhangjiajie railway station 18:24 18:38 8h38m 523 km
12 - Jishou railway station 20:20 20:26 10h26m 648 km
13 - Huaihua railway station 21:53 22:07 12h7m 751 km
14 - Huitong railway station 23:23 23:25 13h25m 841 km
15 - Jingzhou railway station 23:59 00:05 14h5m 880 km
16 - SJ County railway station 01:32 01:41 15h41m 971 km

Yes, you are right. The train connection exists. Sally may make an incorrect search. Now trains run between Zhangjiajie and SJ County - no. 2011 and no. K1473. You can find the exact schedule on the train page of this website.

Hi Marie Jose,
I'm planning the same trip : Zhangjiajie Nat park to SJ.
How did you end up? what train / bus did you take?
Any tips /suggestions?
Thanks,
Cees

You can take a train from ZJJ Railway Station and get to Sanjiang County directly. However, the only schedule is at 17:26. The duration is nearly 8 hours. The ticket fare of soft sleeper is CNY174.5, hard sleeper is CNY114.5 and hard seat is CNY56.5.