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  3. Electric Traction It is generally held that the most efficient method of railway operation, and ultimately the most economica

Electric Traction It is generally held that the most efficient method of railway operation, and ultimately the most economica

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问题    Electric Traction
   It is generally held that the most efficient method of railway operation, and ultimately the most economical, given a reasonably cheap electricity supply, is with electricity as the motive power. The electric locomotive is not dependent, like its steam counterpart, on the competence of driving and firing or the quality of the fuel burned. On the other hand, the speeds that will be possible with any given load are completely predictable and apart from signal or permanent way checks, or other delay-producing casualties, exact observance of schedule times can be guaranteed within narrow limits. With such accurate timetable observance trains can be operated with shorter turnaround times at terminals, which means that more intensive use can be made of rolling stock than is generally possible with steam power.
   In suburban areas the rapid acceleration afforded by electric motors from rest and in recovery from speed restrictions makes it possible to work trains with very frequent stops at higher speeds and a closer headway than with steam locomotives. Multiple-unit working, which brings the motors of two or more sets of passenger stock, or two of more locomotives, under the control of one motorman in the leading driving cabin, would be impossible with steam, for each steam locomotive must be manned by its own driver and fireman. Moreover, except on the fastest and heaviest passenger and freight duties, on which it is advisable to have a second man at the head end to assist in the observation of signals and, if necessary, to attend to the electrical equipment, the equivalent of the steam locomotive fireman is unnecessary; thus the great majority of electric trains have a driver only, which makes for considerable economies in staffing.
   So much for the credit side of the ledger with electric operation. On the debit side is the extremely high cost of providing all the line equipment for supplying current to the trains, and of the sub-stations for feeding the current to the line conductors, hitherto this has been the chief obstacle to more widespread electrification. There is also the obvious disadvantage that, unlike the steam or diesel or diesel-electric locomotive, the electric motive-power unit cannot run anywhere beyond the line or lines equipped with conductors. Concentration of the power supply in large power-stations, also, means that any serious breakdown in the supply can have very widespread effects, bringing a large number of trains to a stand, though with the help of the modern grid system of current distribution in a country like Britain alternative sources of supply can generally be made available without much delay. There is the final disadvantage that in hard winter conditions icing of the conductors, which hinders the picking up of the current, may cause serious delays to trains, as has often been the case on winter mornings on the suburban lines round London; but such trouble is confined mainly to the railways on which the current is picked up from a third rail rather than from overhead conductors. These lines have to be provided with de-icing trains, which patrol them in the early mornings when icing conditions are severe, spreading certain liquid compounds on the rails in order to melt ice formations.
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答案 电力牵引 一般认为，如果能得到合理的廉价电源，以电能为动力便是铁路运输最有效的、归根结底也是最经济的方式。电力机车并不像蒸汽机车那样，它不依靠驱动和点火能力或所用燃料的质量。此外，电力机车在装载任何一定负荷时，其可能行驶的速度完全可以预测，除了信号和线路检查或者严重生产事故引起的晚点之外，能够严格遵守行车时间。由于能准确地按列车时刻表行车，列车在各车站的周转时间缩短了。因此，电力牵引与蒸汽牵引比较，前者更能充分利用机车车辆。 在郊区，电力机车由于电动机启动后能迅速加速，并能从限制速度迅速恢复到正常速度，所以与蒸汽机车比较，在频繁停站的情况下，行车速度较高，而且连发列车的间隔距离较短。当使用带电机的多节车组的列车时，两组或多组客车的电机或是两台或多台机车，均由在驾驶室的一个司机来控制操作，而蒸汽机车则不可能如此，因为每个蒸汽机车必须由自己的驾驶员和司炉工操作；此外，只有在速度最快，载重量最大的客、货车上，电力机车上才需要有第二个人帮助观察信号，并在需要时，照应一下电气设备，除此之外，并不需要有如蒸汽机车的司炉工。因此，大多数电气列车只有一个驾驶员，这就节省了相当多的人力。 电力列车的优点就谈到这里。其缺点方面则是面向列车输送电流的线路设备和向导电线提供电流的变电站的费用非常之高。因此，直到现在，这个问题一直是更广泛地实现电气化的主要障碍。还有一个明显的缺点，不像蒸汽机车、内燃机车或电传动内燃机车，电气机车离开了导电线路就不能行驶。而且电源集中在大电站也意味着电源的任何严重故障可能造成很大的影响，使许多列车停驶。不过在英国这样的国家，用现代化配电网配电，一般可以迅速使用备用电源。最后一个缺点是：在严冬，导线上的结冰会妨碍将电源接上车，从而使列车严重误点，冬天的早晨，伦敦郊区铁路通常就是这样。但是这种问题主要产生于从第三轨而不是从高架线得到电力的那些电路。因此，在这些线路上必须有除冰车。冰冻的早晨，破冰车在线路上巡道，把一些液态化合物撒在轨道上，以便让冰层融化。
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