Your comparing apples to oranges. The Boring Company number is maximum capacity (“able to carry”) while the Metro Madrid number you calculate is an average of a

Your comparing apples to oranges. The Boring Company number is maximum capacity (“able to carry”) while the Metro Madrid number you calculate is an average of actual ridership which will be much, much lower because they only operate at maximum capacity some of the time and even then only in one direction. A maximum capacity 7,500 an hour or even 15,000 is well below some subway lines which have maximum hourly capacity 50,000+ passengers in both directions. But they never achieve that in practice because only one direction is ever at maximum capacity and never average that because transit lines mostly run below maximum capacity.