有三辆小火车，小火车各自分别沿参数指定方向行驶在自己的闭合轨道上。当运行到公共轨道时 需监测是否会有冲突发生，如当火车A在公共轨道上运行的时候，另一辆火车B如果也想进入公共轨道，则必须等待，直到列车A离开了公共轨道，让出该段轨道。为了防止两辆火车在公共轨道上相撞，必须要有一个中央控制系统来调度火车的运行。(There are three small trains, each of which runs on its own closed track in the direction specified by the parameters. If the train A runs in the public orbit, if the train B wants to enter the public orbit, the other train must wait until the train A leaves the public orbit and gives out the track. In order to prevent two trains from colliding on public tracks, a central control system must be provided to control the operation of trains.)