How soon do I need to arrive before my train is due to leave?

I would say 45 minutes to an hour and a half or so. If you have checked baggage, you need to check it by 45 minutes before departure. If your carry on bags exceed the limit, then I'd allow some leeway in case you have to repack

45 minutes to an hour. I think they start boarding about 15 minutes before departure.

Hi Dawn if you are completely ready then i suggest you should reach 45 min before. Reason is they make queue. Depend on number of passengers. This is my personal experience. As I travelled first time from this station.