Because of Fresnel diffraction. See also Fresnel zone.

At a given distance from the wall, the noise intensity decays exponentially with the distance down from the height of the barrier. So each metre of additional wall height helps. A lot.

I distribute. You re-distribute. He gives your hard-earned money to lazy scroungers. -- JakeS

Good point, more relevant than my pantograph noise wild guess (see upthread).

a removable wall that sits close to the rail and needs only a height of 70cm.

Advance notice: last week, I was at the InnoTrans in Berlin (the world's biggest rail trade fair, held every two years), on which I shall report in a diary (hopefully by this weekend). There, I saw what looked like a further development of the system shown in the article you link: there is that noise-absorbing part (which is 55 cm high above the top of the rail and doubles as walkway for track workers) and there are parabolic noise-reflecting acrylic glass railings on the outer side.

_{*Lunatic*, n.
One whose delusions are out of fashion.}