Am back to give a bit of feedback having just secured a seat on the return trip with the Alishan Forest Railway.

I was not successful when the rail tickets were first released. I refreshed the general booking page https://afrts.forest.gov.tw/OT01_1.aspx at 6am but at the point the web page reopened it reverted to Mandarin, selecting English took extra seconds so there were no tickets by the time I put the details in for the Alishan-Chiayi trip. I was gutted.

Then, 2 and a half hours later I had a compulsion to try again and, great luck! I got my ticket (and by the window). I am sure it helped that I just needed the one seat.
Am elated!