As far as I know, there is no direct train from Hanoi to Guilin. You need to connect via Nanning.

Do you know where I can find a timetable for trains from Hanoi to Nanning? Is it a daily train? How long does the whole journey (hanoi to guilin) take?

Thanks!

Twice a week have a train from Hanoi to Russia, pass Guilin & Beijing, but it take about 20-24 hours(from first day 6:30pm to second day 4:00pm) and expensive , you shouldn’t take train , so tired!
You should do like that:
5am catch the small bus from Hanoi to Langson-Huu Nghi Quan (everyday has a lot small bus from Hanoi to Langson) ~ 5$ and it take 4 hours
9:30am to Lang son - Huu Nghi Quan
get in China - You Yi Guan (Chinese language) 30minutes ~ 10:00am
China time is 11:00am
take a taxi from YouYiGuan to PingXiang , just ~ 30RMB ~ 4,5$-5$ about 20 minutes.
11:30am arrive PingXiang - Pingxiang bus station (Pingxiang-qichezhan)
12:00am have lunch near by Pingxiang bus station 6-8RMB ~ 2$
12:40am buy ticlket from Pingxiang to Nanning ~ 66RMB ~ 11$
This bus exactly 2 hours 40 minutes arrive Nanning!
3:20pm to Nanning - Nanning Langdong bus station (Nanning Langdong qichezhan)
You should quickly buy ticket from Nanning to Guilin ~ 108RMB ~ 18$ because there is a lot people here and be careful...be stole!
4:00pm bus start from Nannning to Guilin
(about 5 hours because the highway now fix again)
9:00pm arrive Guilin , take a taxi to your hotel about 15-30RMB
not too late , Guilin night is best time in a day to get out hotel!
Good luck & Enjoy