I don't speak French very well, but my guess is these people are excited about how fast this train is going.  It goes to prove that language is no barrier for railfans, speed aficionados, or people who just like to watch world records being broken.

The TGV - train à grande vitesse - is France's solution to cutting down the time it takes to travel by rail and the time it takes to care for survivors of train wrecks.  Travelling at 574.8 KMH drastically shortens both.