Approaches to the Northern Capital have neither challenging geology nor huge water bodies, therefore Sections 7 & 8 of Moscow - St. Petersburg Highway embodied mainly well-known, proven technological solutions. But some unique construction technologies had to be implemented anyway. The Highway intersects railway tracks of Warsaw direction at km 677. Railway tracks at this section are placed onto an embankment of 15.5 m in height. That means that construction of a bridge above the tracks would create the one of hundreds meters long, or a shorter one but with high approach embankments, and this is an expensive way. Construction of a crossing in the embankment body using an open-cut method is no way either, because the railway is heavily used, and there is no alternative route. The only way was to construct the tunnel under the operating railway. Despite of the tunnels’ tiny dimensions (length of the structure is only 75 m), we had to rack our brains.
https://www.stroygaz.ru/publication/item/mal-tonnel-da-slozhen/