In DC rail transit systems, the running rails are usually used as the return conductor for traction current.
城市轨道交通的供电系统一般采用直流供电，并以轨道作为回流的通路。
When these folks want to return, they simply wave a white flag and the conductor stops the train for them.
当村民们想要搭乘火车时只要挥动一面白旗，售票员就会让火车停下来。
Yielding strength test: Cable shall not cause circuit breakdown, and cause short circuit between the conductor after tested by 15000 times return movement i. e. 30000 times of single route movement.
曲挠试验：在试验期间经15000次往复运动，即30000次单程运动后不发生电流断路，也不发生导体之间的短路。
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