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Why are there several subway lines operated by MTR in Beijing?

Hello, when Metro Line 4 was built, the Jinggang subway was hosted (or something was forgotten), and then it certainly wasn't always. The MTR has only been operating for decades.

Of course, there are many branches in Beijing subway operation.

For example, Beijing Metro operates the first branch, the second branch and so on.

If it is solved, please adopt it ~