Travelers are told to arrive an hour or two early for a flight, but the same principle does not apply to European trains. The departure platform for your train, whether it's a local or a high-speed long distance one, will not be indicated until about fifteen minutes before boarding time. You must hustle to reach the correct platform after it's posted, but arriving at the station much more than a half hour before the departure time is pointless and unnecessary.
Note, however, that if you are using Eurostar, you must be at the station to check-in, thirty minutes ahead of time. Eurostar is the only train I know of where you must check-in and then wait in their amenities-equipped (food, toilets, shops are available) area for the train.