When asked why steam railways are being replaced by electric ones, you will very often get the answer: "to avoid smoke". In recent times, with the growing interest in technical and economic issues, this answer is heard much less often. However, not everyone is aware that electrification is driven more by economic than technical reasons. The superiority of electric traction over steam traction is commonly cited as facts that are easy to see with superficial argumentation, such as: the greater speed of electric trains, the saving of coal, the ease of operation, the regularity of the traffic, and the convenience of travelers.
These are undoubtedly very important factors, but the most important is the economic factor - electric railways are cheaper to run than steam ones and require far fewer staff to operate. I will explain this matter in more detail. When the electric motor was invented in the first half of the 19th century, the inventors of that time began to think about the possibility of using it for transport. In 1834 the first attempt to move a boat with an electric motor powered by electric cells took place on the Neva river near St Petersburg. The first successful application of electric motors for passenger transport took place in Berlin in 1879, where the Siemens company demonstrated a train consisting of trolleys pulled by a small electric locomotive. Electricity was soon supplied by a network suspended over the track. Successful attempts to apply electricity to vehicles encouraged urban transport companies all over the world to introduce electric propulsion in trams. Within a short period of time, the electric tram was the dominant mode of transport in larger cities. On the railways, electric traction did not develop as rapidly due to the difficulty of transmitting energy over longer distances. The direct current of up to 700 volts, which was common at the time, could not be used on main line railways. Thus Italy, which in 1897 was the first to electrify its railways to save imported coal, used alternating current (three-phase system). 
[001] The first electric train, presented by Siemens in 1879.
In the following years, the difficulties of transmitting electricity over long distances were overcome and electrification of the railways began to develop increasingly in many countries. Switzerland, a country without coal and with an abundance of energy from falling water, led the way. At present Switzerland has left steam traction on only a few secondary lines. In the period before the First World War, Germany, France, Austria, Great Britain, Sweden and the United States also had a more significant electric railway network. In the period between the wars, the USSR began electrifying its railways. Let us briefly discuss the operation of electric vehicles running on rails. As we know, a vehicle has much less resistance when running on rails than when running on a dirt road, and less energy is needed to transport a certain load by rail than by road. However, there must be a certain amount of friction between the wheels of the locomotive and the rails, otherwise the wheels start to turn in place when the locomotive is stationary. This sometimes occurs when the rails are particularly slippery. In order to obtain sufficient friction between the wheels and the rails, the locomotive must have sufficient weight. This weight, on the other hand, must not exceed that permitted by the strength of the railway sleepers. The best case is to have a locomotive with such a weight, which is equal to the product of the permissible wheel pressure on the rails by the number of driving wheels (i.e. driven by the engine). This relation is sometimes easily obtained with electric locomotives, which are lighter than steam locomotives, so that they often have to be additionally loaded. 
[002] One of the first electric locomotives, the Swiss Krokodil Be 6/8. A steam locomotive moves by the action of steam produced in a boiler. The steam presses on pistons which turn the wheels on either side of the steam locomotive. The operation of an electric locomotive is based on a different principle. The rotational movement is given by electric motors coupled to the axles by means of gears, since the rotation of the electric motor is always higher than that of the locomotive axles. The electric motor is supported on the locomotive frame by a spring support. The movement of an electric locomotive is therefore much quieter than that of a steam locomotive, as there are no longitudinal movements; moreover, there are no forces acting perpendicular to the track axis, as in a steam locomotive due to the pressure of the pistons. One piston is pressed at a different time from the other. Electricity must be supplied to the engines from outside. This is usually done by connecting the engine on one side to a rail and on the other to a copper or aluminium wire suspended above the track axis. We distinguish between two basic types of electric locomotives: the proper locomotive and the electric driving wagon. In the latter, the engines are placed under the floor of the wagon, and its interior is used to accommodate passengers, while in a locomotive the engines take up part of the interior of the wagon. Motor rail cars can be used when the engines are small in size, usually on light trains. Heavier trains have to be pulled by much larger engines, which also take up more space, in which case the whole wagon is used for electrical equipment. A motive power unit is only capable of pulling one or two, or in exceptional cases three, trailer wagons and can only be used for certain passenger trains. An electric locomotive, on the other hand, can pull any heavy and long passenger and cargo trains, the weight of which depends solely on the breaking strength of the coupling linking the locomotive and the first carriage when the train is climbing hills. Up to this limit, the power of an electric locomotive can be increased by increasing the power of its engines or the number of engines. However, this last characteristic also depends on the number of axles of the locomotive. So if we know the weight of the train, which the designated locomotive is to pull, we can on the one hand calculate the power of its engines, on the other hand (after taking into account the required friction resistance of the wheels against the rails) find the weight of the locomotive. Since we know the permissible wheel (axle) load on the rails, so by dividing the weight of the locomotive by the value of this load we get the number of driving wheels (axles), and thus the number of engines. Electric locomotives usually have axles driven each by a separate motor. 
[003] Czech electric driving wagon (tram) T4 on the streets of Leipzig.
Due to series production and maintenance conditions of the rolling stock, the introduction of as few vehicle types as possible should be sought. On most electric lines the four-engined locomotive type was adopted as the basic type. For heavy freight and express trains a six-engine variety is used. Since steam traction requires the use of at least several, if not more than a dozen types of steam locomotives - this is an additional advantage of the electric locomotive over the steam one. Electricity flows into the locomotive through a contact wire suspended above the track, usually made of copper, although recently aluminium wires have also been introduced. A current collector, pressed against the wire by a spring, called a pantograph, slides along the wire. From the collector the electric current flows to a device called an adjuster, on which is a crank with which the motorman regulates the speed of the train. The current then passes through the motors, flows down to the axles and wheels of the locomotive and returns via the rails to the source of current. The electric motors of the locomotive must be able to regulate the speed within wide limits, as the speed of the train depends on this. Commutator motors are used for this purpose. When used in traction, they work well when supplied with low frequency DC or AC current (usually 16 2/3 Hz). Powering these motors with normal 50Hz current causes considerable sparking and this difficulty has not yet been overcome. Commutator motors have the property that their power consumption decreases significantly as the number of revolutions increases. This phenomenon automatically regulates power consumption when driving on significant hills. This consumption increases as the number of engine revolutions decreases, and so does the speed of the train. On small inclines, the decrease in speed is negligible.
When starting a stationary train, the current consumption would be too high if the motors were switched on directly from the mains. This is why voltage reduction is used during start-up, which reduces the number of motor revolutions obtained for a given current consumption. The voltage can be reduced by connecting the motors in series (this reduces the voltage on the motor poles as many times as there are motors connected in series). In addition, resistors are used, which are incorporated into the current circuit. In practice, both systems are used: a four-motor locomotive for the voltage of 3000V used has two sets of two motors for 1500V each. These sets can be switched in series, i.e. applying 750V to each engine, or in parallel, with each engine running at 1500V. In order to further regulate the voltage, resistors are switched on at both circuits; these resistors are constructed in such a way that some of the resistance wires can be switched off, thus grading the resistance switched on from zero to the highest value when all the resistance wires are switched on. It can be observed that the current runs through only half of the resistor. Switching the resistors on and off is done by the driver turning the adjuster crank. A four-engine locomotive is therefore started in such a way that initially all the engines are switched on in series and the current runs through all the resistors, then the resistors are gradually switched off until they reach zero, then the engines are switched to parallel running with simultaneous switching on of the full resistance, which is then also gradually switched off. Normal train running is carried out with parallel connection without resistance. To further increase the speed, some of the coils in the motor electromagnet windings are short-circuited, which weakens the motor magnetic flux generated by them. With the weakened flux the number of revolutions per minute of the engine increases. In the case of a six-engine locomotive an intermediate connection of motors is used (six in series, three in two parallel sets and two in three sets). 
[004] Electric locomotive power supply. 1. Pantograph 2. Sliding contact 3. Series connection of motors 4. Parallel connection of motors 5. Railway track
Building a locomotive or motor wagon does not yet solve all the technical difficulties. Electricity has to be transmitted from the power station to the grid above the tracks. A direct connection between this system and the power station is not possible due to the long distances involved (it would require large cross-sections of power cables). It is therefore preferable to transmit this energy via high voltage lines to so-called substations built along the track and to convert it there into the voltage needed to supply the traction motors. In the case of alternating current, a first conversion may be carried out at the substation, followed by a secondary conversion in transformers set up on the locomotive itself. This allows the voltage on the overhead line to be raised above the tracks, thus reducing the cross-section of wire and the weight of the entire overhead line. Direct current locomotives must immediately receive power at the appropriate voltage, as this is supplied as alternating current to the substation, where the voltage is lowered and the current converted to direct current in mercury rectifiers. The latter cannot be set up on locomotives. Thus the voltage on DC traction lines is usually 1500 V or 3000 V (when the potential of the rails and earth is zero). With alternating current, voltages of up to 15 000V are used. Because electric trains have high speeds, over 200 km/h, the wire above the track must be suspended at a constant height to avoid pantograph vibrations and sparks. The cable is therefore suspended from the supporting wire by means of vertical joints of various lengths. In addition, both the cable and the wire must be suspended elastically to allow movement along the track as it expands under the effects of temperature. In order to maintain constant tension, the wire and cable are terminated at various intervals by a suitable chain with a weight that hangs downwards. The devices for providing the elastic grids are quite complicated especially at turnouts. 
[005] Railway traction near Rothenbrunnen, Switzerland.
Not only the overhead line, but also the return line requires special care. The rails of the electric railway are connected to each other by a special cable, because if the electrical connection between one rail and the other is insufficient, stray currents can arise. These currents run in the ground independently of the rails and when they meet gas or sewage pipes, they cause the formation of electrolysis cavities on their surface. If the cross-section of the rails proves to be too small, parallel underground return cables are used. The cables are connected to the other pole in the substation. Traction substations, especially with direct current, are very complicated both because of the design of the traction rectifiers themselves and, above all, because of the total automation of the substation operated from a distance. The mercury rectifier is a device which passes the current in one direction only, so it causes the current to flow only half the time when the direction of the current agrees with the operation of the rectifier. In order to give the current the character of a direct current, a considerable number of phases (usually 6 or 12) are used, which, shifted in time with respect to each other, give a total current flowing in one direction and showing only slight oscillations. Leaving aside a whole series of additional devices associated with electric traction, we will finally deal with the conditions of its use on main line railways. In the first place, suburban sections around large cities are electrified. An electric suburban train has a considerable advantage over a steam train in its ability to accelerate rapidly from a station. A steam train, on the other hand, requires a considerable distance to achieve the necessary speed under the same conditions. With a high density of stations in suburban areas, a steam train cannot develop the necessary speed and as a result takes much longer to transport passengers than an electric train. An additional advantage of the latter in suburban traffic is the possibility of varying the number of units, and therefore the number of carriages per day. This makes it possible to establish a rigid timetable, i.e. to run trains at fixed intervals. At the same time it is possible to adjust the number of wagons in these trains to changes in traffic intensity at different times of the day. A number of additional advantages of electric traction, such as the comfort of travel and the cleanliness of the carriages encourage travellers to use this means of transport. 
[006] German electric locomotive, DB103.
The electrification of main line railways outside of suburban sections is a more difficult issue as it requires a great deal of investment, materials and labour. The speed of electric trains over long distances is also higher than that of steam trains, even though in both cases the locomotives can develop the same maximum speed. This is due to both higher accelerations at take-off and lower speed drops on hills. As the average speed of an electric train is higher than that of a steam train, because of the extra stops for water and coal, an electric train will travel a much longer distance in the same amount of time. Taking into account the constant readiness of the electric locomotive to work (no time is lost in firing up the boiler) and the much shorter time needed for repairs and inspections, we will conclude that the electric locomotive can be used to service more trains in a given time. Experience shows that one electric locomotive replaces 2-2.5 steam locomotives. In a similar ratio, the number of necessary wagons and train attendants will be reduced, since the latter, after electrification, will serve more trains at the same time in view of the reduced running time. In addition, one-man operation of electric locomotives can be used. The saving of coal, of course, should not be forgotten, although it is not so serious. The efficiency of a steam locomotive is considerably lower than that of an electric machine in a power station, but with electric traction a certain amount of energy is additionally lost for its transmission in the high-voltage network and in the traction line.The saving lies primarily not in the number of tons of coal burned with both traction systems, but in the possibility of burning inferior kinds of coal, mainly fines, and using brown coal, peat, or the energy of falling water in power stations, which is impossible with steam locomotives, which require better kinds of fuel. Electric traction offers considerable savings in terms of operation, materials and labour, but requires a large one-off investment. These investments pay off where the savings will be greaterv particularly in dense traffic. Since the greatest savings are achieved by electrifying heavy cargo traffic, this usually determines whether a line is ready for electrification. Electrification of railways increases the capacity of railway lines to pass trains, not only because of the greater speed of electric trains, but also because of the smaller difference in speed between passenger and cargo trains, removing the need to stop the latter to pass faster trains. The electrification of railways often makes it possible to postpone until a later date the expansion of the railway infrastructure, which could no longer cope with the increased traffic. All these factors, not least the economic factor, mean that old steam locomotives will probably remain a technical relic in museums and old depots for all eternity. • • •
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