Electric-traction systems can be broadly divided into those using alternating current and those using direct current. With direct current, the most popular line voltages for overhead wire supply systems have been 1,500 and 3,000. Third-rail systems are predominantly in the 600–750-volt range. The disadvantages of direct current are that expensive substations are required at frequent intervals and the overhead wire or third rail must be relatively large and heavy. The low-voltage, series-wound, direct-current motor is well suited to railroad traction, being simple to construct and easy to control. Until the late 20th century it was universally employed in electric and diesel-electric traction units.

The potential advantages of using alternating instead of direct current prompted early experiments and applications of this system. With alternating current, especially with relatively high overhead-wire voltages (10,000 volts or above), fewer substations are required, and the lighter overhead current supply wire that can be used correspondingly reduces the weight of structures needed to support it, to the further benefit of capital costs of electrification. In the early decades of high-voltage alternating current electrification, available alternating-current motors were not suitable for operation with alternating current of the standard commercial or industrial frequencies (50 hertz [cycles per second] in Europe; 60 hertz in the United States and parts of Japan). It was necessary to use a lower frequency (16 2/3 hertz is common in Europe; 25 hertz in the United States); this in turn required either special railroad power plants to generate alternating current at the required frequency or frequency-conversion equipment to change the available commercial frequency into the railroad frequency.

Nevertheless, alternating-current supply systems at 16 2/3 hertz became the standard on several European railroads, such as Austria, Germany, and Switzerland, where electrification began before World War II. Several main-line electrifications in the eastern United States were built using 25-hertz alternating current, which survives in the Northeast Corridor operated by Amtrak.

Interest in using commercial-frequency alternating current in the overhead wire continued, however; and in 1933 experiments were carried out in both Hungary and Germany. The German State Railways electrified its Höllenthal branch at 20,000 volts, 50 hertz.

In 1945 Louis Armand, former president of the French railroads, went ahead with further development of this system and converted a line between Aix-Les-Bains and La Roche-sur-Foron for the first practical experiments. This was so successful that the 25,000-volt, 50- or 60-hertz system has become virtually the standard for new main-line electrification systems.

With commercial-frequency, alternating-current systems, there are two practical ways of taking power to the locomotive driving wheels: (1) by a rotary converter or static rectifier on the locomotive to convert the alternating-current supply into direct current at low voltage to drive standard direct-current traction motors and (2) by a converter system to produce variable-frequency current to drive alternating-current motors. The first method, using nonmechanical rectifiers, was standard practice until the end of the 1970s.

The power-to-weight ratios obtainable with electric traction units had been greatly increased by the end of World War II. Reduction in the bulk of on-board electric apparatus and motors, coupled in the latter with a simultaneous rise in attainable power output, enabled Swiss production for the Bern-Lötschberg-Simplon Railway in 1944 of a 4,000-horsepower locomotive weighing only 80,000 kg (176,370 pounds). Its four axles were all motored. There was no longer need of nonmotorized axles to keep weight on each wheel-set within limits acceptable by the track.

By 1960 the electric industry was producing transformer and rectifier packages slim enough to fit under the frames of a motored urban rapid-transit car, thereby making almost its entire body available for passenger seating. This helped to accelerate and expand the industrialized world’s electrification of metropolitan railway networks for operation by self-powered train-sets (i.e., with some or all vehicles motored). A virtue of the self-powered train-set principle is its easy adaptation to peaks of traffic demand. When two or more sets are coupled, the additional sets have the extra needed traction power. With both electric and diesel traction it is simple to interconnect electrically the power and braking controls of all the train-sets so that the train they form can be driven from a single cab. Because of this facility such train-sets are widely known as multiple-units. Modern multiple-units are increasingly fitted with automatic couplers that combine a draft function with connection of all power, braking, and other control circuits between two train-sets; this is achieved by automatic engagement, when couplers interlock, of a nest of electric contacts built into each coupler head.

From about 1960 major advances in electric traction accrued from the application of electronics. Particularly significant was the perfection of the semiconductor thyristor, or “chopper,” control of current supply to motors. The thyristor—a rapid-action, high-power switch with which the “on” and “off” periods of each cycle can be fractionally varied—achieved smoothly graduated application of voltage to traction motors. Besides eliminating wear-prone parts and greatly improving an electric traction unit’s adhesion, thyristor control also reduced current consumption.

Three-phase alternating-current motor traction became practicable in the 1980s. With electronics it was possible to compress to manageable weight and size the complex equipment needed to transmute the overhead wire or third-rail current to a supply of variable voltage and frequency suitable for feeding to three-phase alternating-current motors. For railroad traction the alternating-current motor is preferable to a direct-current machine on several counts. It is an induction motor with a squirrel-cage rotor (that is, solid conductors in the slots are shorted together by end rings), and it has no commutators or brushes and no mechanically contacting parts except bearings, so that it is much simpler to maintain and more reliable. It is more compact than a direct motor, so more power is obtainable for a specified motor size and weight; the 6,000-kg (14,000-pound) alternating-current motor in each truck of a modern French National Railways electric locomotive delivers a continuous 3,750 horsepower.

The torque of an alternating-current motor increases with speed, whereas that of a direct-current motor is initially high and falls with rising speed; consequently, the alternating-current motor offers superior adhesion for acceleration of heavy trainloads. Finally, the alternating-current motor is more easily switched into a generating mode to act as a dynamic (rheostatic) or regenerative vehicle brake. (In dynamic braking the current generated to oppose the train’s momentum is dissipated through on-board resistances. In regenerative braking, adopted on mountain or intensively operated urban lines where the surplus current can be readily taken up by other trains, it is fed back into the overhead wire or third rail.) The drawbacks of three-phase alternating-current traction are the intricacy of the on-board electrical equipment needed to convert the current supply before it reaches the motors and its higher capital cost by comparison with direct-current motor systems.

A separate traction motor normally serves each axle via a suitably geared drive. For simplicity of final drive it was for many years standard practice to mount the traction motors on a locomotive’s axles. As train speeds rose, it became increasingly important to limit the impact on the track of unsprung masses. Now motors are either suspended within a locomotive’s trucks or, in the case of some high-speed units, suspended from the locomotive’s body and linked to the axles’ final drive gearboxes by flexible drive shafts.

The direct-current motor’s torque:speed characteristics make a locomotive designed for fast passenger trains, whether electric or diesel-electric, generally unsuitable for freight train work. The heavier loads of the latter require different gearing of the final drives—which will reduce maximum speed—and possibly an increase in the number of motored axles, for increased adhesion. But considerable mixed-traffic haulage capability is obtainable with three-phase alternating-current motors because of their superior adhesion characteristics.

Direct-current motor technology was employed in Japan’s first Shinkansen and France’s first Paris-Lyon TGV trains, but by the early 1990s three-phase alternating-current traction had been adopted for both Japanese and European very-high-speed train-sets—and by extension the systems around the world that have been derived from them. In Europe, international train operation without a locomotive change at frontiers is complicated by the railways’ historic adoption of different electrification systems, either 1,500 or 3,000 volts direct current or 25,000 volts 50 hertz or 15,000 volts 16 2/3 hertz alternating current. For instance, TGV-type trains could not operatie at full efficiency between London, Paris, and Brussels on the Eurostar line via the Channel Tunnel as long as they had to accommodate French 25,000-volt alternating-current overhead wire, Belgian 3,000-volt direct-current overhead wire, and British 750-volt third-rail supply. The French had perfected traction units capable of operating on more than one voltage system soon after they decided to adopt 25,000-volt alternating-current electrification in areas not wired at their previous 1,500-volt direct current. Nevertheless, where very-high-speed traction was concerned, it was impossible to contain within acceptable locomotive weight limits the equipment needed for equivalent high-power output under each system. Only after all the new high-speed lines were electrified on high-voltage alternating current was a true high-speed service available on the Eurostar line.

Since about 1980 the performance and economy of both electric and diesel traction units have been considerably advanced by the interposition between driving controls and vital components of microprocessors, which ensure that the components respond with maximum efficiency and that they are not inadvertently overtaxed. Another product of the application of electronics to controls is that in the modern electric locomotive the engine operator can set the train speed he wishes to reach or maintain, and the traction equipment will automatically apply or vary the appropriate power to the motors, taking account of train weight and track gradient. The microprocessors also serve a diagnostic function, continuously monitoring the state of the systems they control for signs of incipient or actual fault. The microprocessors are linked to a main on-board computer that instantly reports the nature and location of an actual or potential malfunction to a visual display in the driving cab, generally with advice for the cab crew on how it might be rectified or its effects temporarily mitigated. The cab display also indicates the effectiveness of the countermeasures taken. The computer automatically stores such data, either for downloading to maintenance staff at the journey’s end or, on a railroad equipped with train-to-ground-installation radio, for immediate transmission to a maintenance establishment so that preparations for repair of a fault are in place as soon as the traction unit ends its run. In newer very-high-speed, fixed-formation train-sets, a through-train fibre-optics transmission system concentrates data from the microprocessor controls—both those of passenger car systems, such as air-conditioning and power-operated entrance doors, and those of the rear locomotive or, in the Japanese Shinkansen train-sets, the traction equipment dispersed among a proportion of its cars.

Diesel traction

By the end of the 1960s, diesel had almost completely superseded steam as the standard railroad motive power on nonelectrified lines around the world. The change came first and most quickly in North America, where, during the 25 years 1935–60 (and especially in the period 1951–60), railroads in the United States completely replaced their steam locomotives.

What caused the diesel to supersede the steam locomotive so rapidly was the pressure of competition from other modes of transport and the continuing rise in wage costs, which forced the railroads to improve their services and adopt every possible measure to increase operating efficiency. Compared with steam, the diesel traction unit had a number of major advantages:

1. It could operate for long periods with no lost time for maintenance; thus, in North America the diesel could operate through on a run of 3,200 km (2,000 miles) or more and then, after servicing, start the return trip. Steam locomotives required extensive servicing after only a few hours’ operation.

2. It used less fuel energy than a steam locomotive, for its thermal efficiency was about four times as great.

3. It could accelerate a train more rapidly and operate at higher sustained speeds with less damage to the track.

In addition, the diesel was superior to the steam locomotive because of its smoother acceleration, greater cleanliness, standardized repair parts, and operating flexibility (a number of diesel units could be combined and run by one operator under multiple-unit control).

The diesel-electric locomotive is, essentially, an electric locomotive that carries its own power plant. Its use, therefore, brings to a railroad some of the advantages of electrification, but without the capital cost of the power distribution and feed-wire system. As compared with an electric locomotive, however, the diesel-electric has an important drawback: since its output is essentially limited to that of its diesel engine, it can develop less horsepower per locomotive unit. Because high horsepower is required for high-speed operation, the diesel is, therefore, less desirable than the electric for high-speed passenger services and very fast freight operations.

Diesel development

Experiments with diesel-engine locomotives and railcars began almost as soon as the diesel engine was patented by the German engineer Rudolf Diesel in 1892. Attempts at building practical locomotives and railcars (for branch-line passenger runs) continued through the 1920s. The first successful diesel switch engine went into service in 1925; “road” locomotives were delivered to the Canadian National and New York Central railroads in 1928. The first really striking results with diesel traction were obtained in Germany in 1933. There, the Fliegende Hamburger, a two-car, streamlined, diesel-electric train, with two 400-horsepower engines, began running between Berlin and Hamburg on a schedule that averaged 124 km (77 miles) per hour. By 1939 most of Germany’s principal cities were interconnected by trains of this kind, scheduled to run at average speeds up to 134.1 km (83.3 miles) per hour between stops.

The next step was to build a separate diesel-electric locomotive unit that could haul any train. In 1935 one such unit was delivered to the Baltimore and Ohio and two to the Santa Fe Railway Company. These were passenger units; the first road freight locomotive, a four-unit, 5,400-horsepower Electro-Motive Division, General Motors Corporation demonstrator, was not built until 1939.

By the end of World War II, the diesel locomotive had become a proven, standardized type of motive power, and it rapidly began to supersede the steam locomotive in North America. In the United States a fleet of 27,000 diesel locomotives proved fully capable of performing more transportation work than the 40,000 steam locomotives they replaced.

After World War II, the use of diesel traction greatly increased throughout the world, though the pace of conversion was generally slower than in the United States.

Elements of the diesel locomotive

Although the diesel engine has been vastly improved in power and performance, the basic principles remain the same: drawing air into the cylinder, compressing it so that its temperature is raised, and then injecting a small quantity of oil into the cylinder. The oil ignites without a spark because of the high temperature. The diesel engine may operate on the two-stroke or four-stroke cycle. Rated operating speeds vary from 350 to 2,000 revolutions per minute, and rated output may be from 10 to 4,000 horsepower. Railroads in the United States use engines in the 1,000-revolutions-per-minute range; in Europe and elsewhere, some manufacturers have favoured more compact engines of 1,500–2,000 revolutions per minute.

Most yard-switching and short-haul locomotives are equipped with diesel engines ranging from 600 to 1,800 horsepower; road units commonly have engines ranging from 2,000 to 4,000 horsepower. Most builders use V-type engines, although in-line types are used on smaller locomotives and for underfloor fitment on railcars and multiple-unit train-sets.

The most commonly employed method of power transmission is electric, to convert the mechanical energy produced by the diesel engine to current for electric traction motors. Through most of the 20th century the universal method was to couple the diesel engine to a direct-current generator, from which, through appropriate controls, the current was fed to the motors. Beginning in the 1970s, the availability of compact semiconductor rectifiers enabled replacement of the direct-current generator by an alternator, which is able to produce more power and is less costly to maintain than an equivalent direct-current machine. For supply of series-wound direct-current traction motors, static rectifiers converted the three-phase alternating-current output of the alternator to direct current. Then in the 1980s European manufacturers began to adopt the three-phase alternating-current motor for diesel-electric traction units seeking advantages similar to those obtainable from this technology in electric traction. This requires the direct-current output from the rectifier to be transmuted by a thyristor-controlled inverter into a three-phase variable voltage and frequency supply for the alternating-current motors.

On some railroads with lightly laid track, generally those with narrow rail gauge, locomotives may still need nonmotored as well as motored axles for acceptable weight and bulk distribution. But the great majority of diesel-electric locomotives now have all axles powered.

Other types of transmissions also are used in diesel locomotives. The hydraulic transmission, which first became quite popular in Germany, is often favoured for diesel railcars and multiple-unit train-sets. It employs a centrifugal pump or impeller driving a turbine in a chamber filled with oil or a similar fluid. The pump, driven by the diesel engine, converts the engine power to kinetic energy in the oil impinging on the turbine blades. The faster the blades move, the less the relative impinging speed of the oil and the faster the locomotive moves.

Mechanical transmission is the simplest type; it is mainly used in very low-power switching locomotives and in low-power diesel railcars. Basically it is a clutch and gearbox similar to those used in automobiles. A hydraulic coupling, in some cases, is used in place of a friction clutch.