Where does it go?The Eurostar runs from London to Paris or Brussels in under 2.5 hours and back, and may stop at Lille, Calais and Ashford. (Take it to Brussels and get a single train ticket from there to Amsterdam for an easy London-Amsterdam trip, too.) Your Eurostar ticket nets a free ride from anywhere in Belgium to the main Midi Brussels train station (a station we're not crazy about -- be careful here) within 24 hours of the Eurostar's departure or arrival. In London, it used to arrive at Waterloo station but now goes to St. Pancras (both are easy tube rides to central London or a good London hostel). Check out a Eurostar map.
Where can I get Eurostar tickets?You can buy Eurostar tickets online before you leave the US (up to four months before travel), and it's not a bad idea -- the train does fill up fast. Your ticket includes a seat reservation, just like an airplane -- and has many of the same restrictions (crummy refund policy, etc.). You may get a Eurostar voucher with your Eurail pass if you're getting one for your European trip -- after you've chosen a Eurail pass, check the details.
How much do Eurostar tickets cost?A one-way youth flexi voucher (below) bought in advance from the US is about $100 or less. A round trip youth ticket bought under restrictions at the station is a bit over $100, and full fare, full boat tickets are about $200 each way. Your Eurail pass will not cover a trip on this train; check the details after you buy a Eurail pass, though -- you may be getting a cool deal on a youth flexi voucher, though, depending on pass and season.
What's a Eurostar youth flexi voucher?Youth flexi vouchers are discount Eurostar tickets good for 60 days; you must be under 26 to get one (learn more about how to get student travel discounts!). It may come with your Eurail pass (see just above). The voucher can be exchanged for a ticket at a London Waterloo train station window, in the Paris Gare du Nord train station or at the Brussels Midi station near the Eurostar terminal.
What's check in like?You check in to a Eurostar train similarly to the way you check in for an airplane flight in the US, except you can carry whatever gels and liquids your heart desires. Arrive about 45 minutes before departure. You'll go through security and your stuff will go through an x-ray. You're supposed to limit your baggage to two big carry ons (your backpack is ok) and one small carry on. You will need to show a landing card on the way into the UK from the EU (more on that below). You'll get your passport stamped.