|The St. Pancras Rail Station in London is a beautiful and imposing building.|
For most European trains, it is necessary to arrive only twenty minutes or so before departure, but that is not true with the Eurostar. Be sure to arrive at the station at least 45 minutes early. You must go through security similar to that at airports and the process takes time. Eurostar requires passengers to arrive at least thirty minutes in advance, but judging by the dozen people who begged to go ahead of us in line for fear of missing their train, a lot of travelers seem to forget the rule.
After passing through security in large stations, you will see a sign that lists the train platforms but do not expect to see the actual platform numbers until about ten minutes before departure. You will also see signs directing you to various approach points depending on your coach assignment. Seats in coaches one and two might take the escalator straight ahead while passengers in coaches nine and ten are directed up a different platform.
In smaller stations like Calais, it is still important to arrive 45 minutes early because you must pass through security, but all the uncertainty is missing regarding platforms because there's only one train and one exit door. Once you are directed to the only platform, signs along the roof tell you where to stand so you can access the proper coach that's been assigned to you.
We traveled second class, but I've got to tell you that, unlike other trains, we found it a bit cramped. We took turns going back to the luggage section between the cars to sit on the jump seat. It was a lot cooler and more spacious there. Besides, as long as we remembered not to fall out by leaning on the glass door, we had a fabulous view!
Remember to save 50% by purchasing your Eurostar tickets early. See this post for details.
Also consider using the Eurostar site itself rather than the US-based Rail Europe to purchase your tickets. (The British-based Rail Europe site is fine though.)
Consider packing a lunch so you won't feel left out when the other passengers unwrap their tasty treats. At the St. Pancras London station, there's a great carry-out store with reasonably priced and good-tasting sandwiches. You will find this far less expensive than buying lunch on the train itself.