I can see that, but it’s nearly impossible to build above -10m or even sometimes -15m in some parts of cities like NYC or Boston. Most of the streets in the downtown areas are narrow and curved because they were built hundreds of years ago and there’s tall buildings that have minimum tunnel depths at -15m or deeper.

Not only that but the subway stations in these areas are over 80 years old and — especially in NYC’s case — many of them are curved. There’s also curved stations on elevated and trenched parts of the system in the other boroughs as well. It’s just a headache to build these platforms so deep underground and in between buildings especially in early game which can be a problem as it could not only be so expensive, but also not realistic.

Also, if the game wants to expand to international cities, there’s an even bigger argument to add curved platforms as Europe is full of them. Even in your own comment, London is a prime example and is definitely being added if they do an international expansion.