Imagine this. Paris started construction of its Metro in the late 1800s and now has 133 miles of track with no new construction. New York: 1863 and 233 miles. London: 1863 and 250 miles.
Istanbul: 1992. When the current construction is finished in 3 years: 400 miles!
The line that crosses UNDER the Bosphorous between Asia and Europe is 1,500 feet underground and the (park and ride) stop "Haciosman" on the north end of the like which goes through the business district ("M2") has a huge bus station on top at ground level, below which is a 5 story parking garage below which is the metro station. It takes 7 long escallators to get from the metro station up to ground level. [Maybe the engineers decided that rather than having to stop construction for archological ruins in this 9,000 year old city, they would just have the lines go under all possible sites.]