Eurostar orders its first double-decker trains! I have ridden on Amtrak double-decker train before (California Zephyr); and you get better less obstructed view on the upper deck.
Interesting that the double deck cars will be just 6 inches taller than the current cars.
Then you will be able to see the tunnel's ceiling.
If they are like the double-decker REs in Germany, the extra space in gleaned from the space between the wheels. You board at platform level and go up stairs to the top level and down stairs, a little, to the lower level.
In the old single level cars, the single level was much higher than the platform.
Remember that the Eurostar brand has recently been extended to also cover the Paris/Brussels/Amsterdam/Koeln trains previously marketed as Thalys - it hasn't been confirmed which routes the new trains will operate on.