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The Shanghai subway system is a good example. Construction started in 1986, first 28 stations opened in 1993.

It now has 413 stations and is the largest subways system by length (676 km) in the world.

Subways drastically vary depending on the material the ground is made of and what is there already. For example London is relatively easy to build deep tunnels in, but on of the big things that has increased the complexity of Crossrail is that central London has a lot of stuff underground that isn't necessarily documented correctly. There are water mains, sewers, electrical cables, ventilation shafts, other subway tunnels, stations, then abandoned instances of all of those and plenty of archeological materials (London as a city has been around for over 2000 years).

True, totally unfair. Shanghai has only been occupied for ~8000 years.

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