我的答案：
1。 首先，电力机车供电方式有overhead wire，third rail， on-board storage device 三种。楼主大概指的是第二种。
2。第二种的工作方式：The third rail is located either in between the two running rails or, usually, on the outside of them. The electricity is transmitted to the train by means of a sliding "shoe" (pick-up or contact shoe) which is held in contact with the rail. On many systems an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side (side running) or bottom (bottom running) of the third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides on top, it is referred to as "top running". When the shoe slides on the bottom it is not affected by the build-up of snow or leaves.
3。那第三种危不危险呢？As with overhead wires, the return current on a third-rail system usually flows through one or both running rails, and leakage to ground is not considered serious.
但是！！！An unguarded electrified rail is a safety hazard, and some people have been killed by touching the rail or by stepping on it while attempting to cross the tracks. However, such incidents are usually the result of carelessness on the part of the victim.
所以，电力机车及其轨道是危险的，请大家远离！
