Wheel wear is a concern on most metro systems.
对大部分地铁系统来说车轮磨损事关重大。
The larger cities have metro systems in operation, under construction, or in the planning stage.
较大的城市有地铁系统在运行，正在建设中，或在规划阶段。
And there will be new metro and transport systems.
A还将有新的地铁和交通系统。
And there will be new metro and transport systems.
A还将有新的地铁和交通系统。
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