Both high speed trains are crossing at 250 Km/h. We can see that the Eurostar trainset is longer than the double deck trainset in backgroung. The Eurostar can be cut in half for safety issue in the Channel Tunnel, the half train becomes autonomous in this case to go out of the tunnel from the side where there is no problem (in case of fire for example) ! The specific Alstom TGV technology is that each passenger car lies with a single and common bogie and each car is "coupled" with those around and form an inseparable train. That the reason because Alstom high speed trainset are the most secure trains in the word. The Eurostar, on the center of the trainset, have cars with double bogies.
