The railway between Britain, France and Belgium via the Channel Tunnel. Currently terminates at Waterloo Station.
Trains depart very regularly (generally every half hour) from about 5am to about 9 or 10pm, and thanks to the opening of the new Channel Tunnel Rail Link (phase 1), take about 2 hours 20 minutes to get from Waterloo to the Gare du Nord in Paris or 2h35 to get to Midi station in Brussels.
Unfortunately for leisure travellers there is a 30-minute check-in restriction (although business travellers get 15 minutes). What does this mean? --Kake
You have to check in (just like an airline) at a desk up to 30 minutes before your train departs... otherwise they generally won't let you on. This is so they can make sure all the passengers have cleared Customs which is at Waterloo (i.e. when you are on the train you are essentially in "France"). --Dave
Eurostar now offer some cheap fares - starting at around �39 return - making it as cheap as, and probably faster than, the plane, since it takes so much time to transfer to and from the airports in London and Paris.