The trip from Paris to London is 20 minutes shorter thanks to a $60 million (35 million-pound) upgrade to the Eurostar's Channel Tunnel line (Buy tickets now). Eurostar makes it possible, in fact very doable, to add a side trip to Paris or London when visiting the other city.
Beforehand, it took nearly three hours to make the crossing, and now it will take two hours, 35 minutes. Further upgrades scheduled to be completed recently knocked another 15 minutes off. The trains will finally travel at their top speed of 186 miles per hour, according to Eurostar.
With speeds this fast and these two cities both so alluring, it is really easy to add a day in London to your Parisian vacation, and vice versa. Although Paris lost its bid for the 2012 Olympics to London, now anyone planning to attend the games can make a quick sidetrip to Paris.
It can also make it possible to have a longer European vacation spanning two (or more) countries. Combine Eurostar with a rail pass and you can hit several cities during your vacation.
For more on Paris and on rail travel in Europe, see these resources: