To build the 225-metre long, 24-metre wide train hall and its even larger piggy-backing ticket hall, it was necessary to excavate a cavity 40-metre wide, 20-metre high and 250-metre long—this 30-metre underneath the busy city centre in unstable waterlogged sedimentary rock. The resulting station is cathedral-like in proportions, with a ticket hall so spacious that there is room for a mezzanine. The entire construction is waterproofed on both sides by a 7-metre thick, 10-metre high abutment of concrete which contains escalators linking the two levels.