Eurostars have two levels for cars actually, but they are fully enclosed and have quite some space on both sides, so it's indeed easier to load and get out (though it's not really needed because the trip is short and there is nothing to do outside of the car). Those trains are much wider than usual though and can only travel on some very special tracks, only the tunnel I think.

I think it's not as much about mesh form as passageways could be filled easily, but width on the other hand can't be changed at all.