Does anyone have any advice they could share on travelling to Luoyang from Hong Kong by train?

No direct train operates between these two places.
You can get to Zhengzhou Railway Station first from HK. The schedule is at 15:15 with the duration of around 18 hours. The ticket fare of hard sleeper is CNY377.

Then take a high speed train to LY City. It takes no more than an hour on the way. And the ticket fare of first class seat is CNY59.5 and second class seat is CNY39.5.