Chapter IV : Powered Rail Vehicles & Locomotives
4.1 Types of Railway Traction Rolling Stock and Their Classification
Railway traction rolling stock can be classified into two groups: non-autonomous and autonomous.
Non-autonomous rolling stock is provided with energy from an outside source, such as electric locomotives and trains, while autonomous rolling stock receives energy from a power plant mounted directly on or inside the vehicle. Autonomous rolling stock has advantages such as lower mass and higher power, reduced environmental effects, and more efficient energy use.
Powered rolling stock are commonly divided into categories of passenger or freight transportation, shunting operations, and poled vehicles for special purposes. Track gauge dimensions vary widely worldwide, and loading gauge covers predefined dimensions for rail vehicles to ensure safe operation.
Vertical axle load limits on a track are determined by the structure of individual rail networks, and the electric power type used in the power traction system.
Maximum power of the powered rail traction vehicle is determined by the effective maximum capacity of the power plant. Maximum operational speed is determined by the type of car body, number of driving cabs, unit configuration, connection type, coupling and absorbing devices (draft gear), number of bogies in a rail-powered vehicle, motor installation, and number of consecutive motorized and non-motorized axles.
The International Union of Railways (UIC) classification of axle arrangements classifies axles within the same bogie by alphabetical symbols for the number of consecutive motorised axles and numerals for the number of consecutive non-motorized axles between motorized axles. The Association of American Railroads (AAR) system uses a minus sign for separation of bogies used in one locomotive and a plus sign for articulated connections between bogies or vehicle sets/train configurations.
Passenger rolling stock is divided into various groups, including light rail vehicles, trams,
locomotives, and motorised carriages. Light rail vehicles are non-autonomous electrically propelled vehicles with steel wheels that run on steel rails and can negotiate sharp curves. They are not constructed to structural criteria like heavy rail vehicles.
Passenger trains consist of motorized passenger cars or locomotives and unpowered passenger cars designed to carry passengers. These are often implemented in diesel multiple unit (DMU) or electric multiple unit (EMU) designs for city, suburban, and regional rail networks
Freight traction rolling stock consists of locomotives designed for high tractive effort for hauling large freight and heavy-haul ore or coal trains. These locomotives can be autonomous or non-autonomous and can be either autonomous or non-autonomous. Heavy-haul locomotives are designated for hauling longer unit trains used to carry large payloads of bulk products. Shunting locomotives perform works in stations related to train formation and can work on track with lesser axle loading.
Traction rolling stock for special purposes include maintenance of way vehicles, military use rail vehicles, and firefighting and rescue vehicles.
4.2 Motive Power Energy Principles
Modern traction rolling stock is divided into four groups based on the type of energy/power supply system: electric, diesel, gas turbine, and hybrid. The first practical prototypes of electric rolling stock were created in the 1920s, and industrial production began in the 1930s. Electric locomotives
experienced progressive improvements until World War II, when demand for them increased due to the repair of damaged sections of railway infrastructure and the creation of new railway corridors.
Electric rail traction vehicles can operate using multiple types of voltages and currents, known as multi-system performance. They consist of electrical, mechanical, pneumatic, and hydraulic systems.
Mechanical systems include the car body, main frame, coupling devices, suspension, devices for transmission of tractive and brake efforts, bogies, and air cooling and ventilation. Pneumatic systems include an air compressor, automatic control system, reservoirs for compressed air storage, control and management units, and dynamic and regenerative braking systems. Electrical equipment includes contact conductors, power transformers, inverters, traction electric motors, auxiliary machines, electrical control and management units, and dynamic and regenerative braking systems.
Light rail vehicles are commonly equipped with standard pneumatic, electric, rail, and/or eddy current brakes. The main disadvantages of electric rail traction vehicles are the high costs of electrified network infrastructure and maintenance, and the lack of fully autonomous operation. Modular design has also been applied to light rail vehicles, with car bodies supported on bogies or supported by other car bodies through special beam and joint connections.
Electric Rail
Electric locomotives are designed to operate on rail networks and are equipped with standard
pneumatic and electric brakes. They can be one-cab or two-cab versions and can operate as multi-unit systems. Multi-system electric locomotives have current collection, traction, and power equipment for working with various combinations of current and voltage.
Electric multiple units (EMUs) are trains used for passenger transportation on city, suburban, and regional rail networks and high-speed passenger trains. They have similar designs to electric
locomotives but are powered trains with driving, motor, and/or trailer cars. EMUs can have a modular design, with a shared bogie approach between adjacent cars.
Suburban EMU trains typically operate at speeds no higher than 180 km/h, providing good train dynamics under high rates of acceleration and braking. They have increased numbers of driven wheels or wheelsets, standard pneumatic and electric brakes, rail brakes, and eddy current brakes. High-speed EMUs can reach speeds of 400 km/h, requiring significant power and new design solutions for reliability and safety. These trains are equipped with active suspension systems, solid wheels, traction motors, drive shafts, and low-level floors. The aerodynamic design is crucial for high-speed trains due to significant drag forces and increased aerodynamic noise and vibrations.
Design Traction
Diesel-powered autonomous traction vehicles are prevalent on rail networks due to their
self-contained nature, ability to operate in any climate zone, and efficient power generation. However, they have environmental concerns and higher maintenance costs compared to electric vehicles.
Diesel locomotives are divided into freight, passenger, mainline, and shunting types. They consist of a power plant and four basic systems: mechanical, electrical, pneumatic, and hydraulic. The car body of a diesel-electric locomotive is divided into operator, auxiliary, alternator, engine, and radiator modules.
The working principle of a locomotive is to convert gas produced by combustion processes into pressure force on pistons, which is converted into rotational energy of the crankshaft. This energy is transferred to the transmission system and transformed into traction motors. Diesel locomotives can be designed as one-cab or two-cab versions, and can operate as multiple-unit systems. Electric
transmission systems provide optimal tractive and economic operating characteristics, while mechanical transmission systems are used for locomotives with low power. Hydraulic transmission systems consist of a hydraulic gearbox connected to the crankshaft and mechanical transmission to the wheelsets, adjusting traction torque by changing the flow rate and pressure of the working liquid (oil).
However, hydraulic transmissions require high-level skills and technical expertise, and have lower efficiency compared to electric transmissions.
Locomotive auxiliary equipment includes cooling, air supply, fuel supply systems, sanding system, fire protection system, electrical auxiliary equipment, and low-voltage circuits. Dual-fuel diesel
locomotives have been introduced in recent years, allowing the locomotive to run on either diesel or liquefied natural gas. Diesel multiple units (DMUs) provide passenger transportation in
non-electrified or partially electrified service areas. They consist of driving, motor, and/or trailer cars in a classic design scheme. Diesel power plants produce energy that is then transferred to the traction motors (traction transmission). Diesel multiple units can be divided into three categories:
Diesel-EMUs, Diesel-mechanical multiple units, and Diesel-hydraulic multiple units.
There are two common locations for the power plant in DMU trains: traditional with the diesel engine installed in the driving car behind the driver cab, and modern DMU trains with the diesel unit placed in underfloor space between the bogies of the motor or driving cars. Gas turbine rail traction vehicles are equipped with a gas turbine as a power plant, but have not found wide application in railway operations. Hybrid traction vehicles are similar to diesel and gas turbine traction vehicles but use electrical energy stored in electric batteries, supercapacitors, or flywheels. Hybridization designs include hybrid design with no internal energy storage and only external storage units, hybrid
construction with internal accumulator units, and complex hybrid structures that combine several types of energy storage. Hybrid locomotives are already in operation for shunting services, suburban and urban passenger traffic, but are not used for freight or heavy-haul operations due to limitations of existing energy storage options.
4.3 Classification of Main Rail Traction Vehicle Components and Suspension Systems Rail traction vehicle dynamics involve the design of components and systems, as well as
understanding their functionalities and principles of work. The main components of a vehicle include the car body, which accommodates equipment, personnel, and passengers, and can be divided into two types: main frame (underframe) and monocoque construction (stressed skin design). The main frame type receives, carries, or carries all main loads, while the monocoque body has rigid link connections and low weight.
The running gear of powered rail vehicles was initially without bogies due to the use of crank mechanisms as a traction transmission. However, the introduction of individually driven wheelsets allowed for the widespread introduction of running gear with bogies. The main purpose of a rail
vehicle bogie is to improve dynamic interaction between the running gear and rails in curved sections of track. Bogies are available in two-axle, three-axle, and four-axle design variants, depending on the design parameters of a rail traction vehicle and restrictions due to loading gauge and axle load.
The main elements of a bogie include the bogie frame, braking system equipment, locomotive sanding system, spring suspension, wheelsets with associated assemblies, and traction drives or wheel-traction drive assembles. Conventional bogies have some clearance in their primary suspension design to improve dynamic interaction between wheels and rails. An alternative bogie design can be equipped with individual drives, where the traction torque from the motor acts on each wheel.
Traction drives are mechanisms that transfer kinematic power from traction motors or mechanical gear transmission to the wheelsets or wheels of a powered rail vehicle. These designs can be individual or grouped, with the former acting on one wheel or one wheel, and the latter sharing traction torque between multiple wheelsets or bogie wheels. The design and parameters of traction drives are often dependent on the installation designs of traction motors and associated gearing. Three design variants have found wide application: nose-suspended traction motor, frame-mounted traction motor, and body-mounted traction motor.
The first design variant has traction drives with one part resting on the axle of the wheelset through rolling or slip bearings and the other connected through elastic-damping suspension to the frame of a bogie or rail vehicle. This design is widely used in rail traction vehicles with relatively low design speed. The other two design variants are similar, with the traction motor mounted to the bogie frame or the main frame, providing the wheelset with torque through mobile and flexible connection elements.
Spring suspension is necessary for a rail vehicle to reduce force interaction with the track and minimize and dampen dynamic forces and natural oscillations. Suspension systems can be performed in several stages, acting in the horizontal, vertical, and transverse planes. Active suspension, equipped with a control system, is used to provide further security and stability of operation with higher traction and braking forces.
4.4 Connection Between a Frame/Car body and Bogies
Connection elements between a frame or car body and bogies are crucial for supporting the rail vehicle car body on bogie frames and transmitting traction and braking forces from the bogies to the car body.
These elements can also form parts of the secondary suspension system. Common connection designs include pivot assemblies, side bearings, links and linkages, return devices, and flexi-coil suspension.
Traction rods are used to transmit traction and braking forces between them. Pivot assemblies can be divided into two types: high and low. When these points have low locations, a rail traction vehicle can achieve higher tractive and brake efforts. Traction rods can also directly connect a car body and a bogie when a powered rail vehicle is not equipped with pivot assemblies.
Traction rods can be equipped with absorbing devices, such as rubber and rubber-metal elements or bushings, for damping oscillations of traction and brake forces.
The classification of rail traction vehicles as AC or DC depends on the type of electricity supplied to their traction motors. Torque production relies on Lorentz force or reluctance forces, with the magnetic component dominating in most traction machines. In DC machines, torque is produced by
the interaction of currents in two windings, such as the field winding and armature or the stator and rotor in an induction machine.
4.5 Traction Systems and Their Classification
Key principles for both AC and DC traction machines are illustrated using a simple model. The force equation determines the limit on machine torque, while the magnetic field strength is determined by the magnetic properties of the machine magnetic materials. For continuous operation, cooling methods are used to determine maximum allowable losses, while short-term operation is determined by the specific heat of the motor materials.
Both AC and DC machines have a characteristic low-speed region where machine torque is adjustable within limits set by the rated maximum field and rated maximum currents. The mechanical power is the product of machine torque and rotational velocity, and the generator equation determines the behavior of electrical traction machines.
The mechanical power delivered by a conductor on an armature or rotor moving at wmech is equal to the electrical power. Mechanical to electrical efficiencies are in the approximate range of 80%-95%
for both AC and DC traction machines and their electronic controls. Simple models can be developed based on the equality of input and output powers and constraints on torque and power in the
low-speed constant-torque region and high-speed or constant-power operational regions.
In a DC rail traction vehicle, the traction machines require DC supplies for both the armature and field windings. High field strengths result in the largest levels of torque production for any level of armature current. At higher speeds, the induced armature voltage must be limited by reducing the field-winding currents, and the maximum torque production falls. This dependency is directly visible in the tractive effort curve of any DC rail traction vehicle.
Common DC rail traction vehicle topologies include diesel-electric rail traction vehicles with an AC-DC topology, which are equipped with a power traction transmission system, main components of pantograph/s, DC:DC converters, choppers, control equipment, and traction motors. Electric rail traction vehicles with an AC-DC topology use DC power from an overhead network but are equipped with DC traction motors, with main components of pantographs, transformers, rectifiers, and traction motors.
In an AC rail traction vehicle, the traction machines are controlled by DC-AC inverters. Two common inverter control strategies are field-oriented control (FOC) and direct torque control (DTC), both offering similar high levels of performance.
Hybrid traction systems combine at least two power sources or energy storage systems, often incorporating energy storage. The most common railway hybrids are those that incorporate energy storage, with the electrical traction machine being most often the induction machine. Energy storage units are most frequently batteries, flywheels, or supercapacitors, with lithium-ion batteries being the most commonly used.
Hybrid vehicles are further classified as weak and strong hybrids, with weak hybrids having limited energy storage and weak hybrids having significant storage.
4.6 Brake Systems and Their Components
Brake systems are essential for maintaining or reducing the operational speed of rail vehicles, whether operating independently or as traction vehicles in a train configuration. They are divided into two groups based on the method used to create resistance force: adhesion and non-adhesion. Adhesion brakes are commonly split into frictional and dynamic systems, with energy absorbed by friction between the wheel and brake shoes, pads, or discs. Dynamic brake systems work based on the transformation of kinetic energy into other types of energy, with the main one being electrical.
Non-adhesion brakes use electromagnetic forces acting on rails to stop or decelerate a rail traction vehicle or a train.
Brake systems can contain several types of brakes at the same time, such as shoes and discs, and vehicles can also be equipped with dynamic, electromagnetic, and rail brakes. The most common braking systems of rail traction vehicles used in rolling stock design are pneumatic systems, which use shoes or wheel disc brakes. A typical air brake system includes feeding and supply components, energy storage components, activation components or actuators, and a mechanical system for transferring braking efforts.
Dynamic braking systems use the absorption of kinetic energy to control the speed of rail traction vehicles. The main type of dynamic brake used on rail vehicles is the electric brake, which converts the kinetic energy of the moving vehicle into electrical energy. DC rail traction vehicles braking is accomplished by providing a power supply to the traction motor fields, which can be controlled by adjusting the field or switching braking resistors in or out of circuit.
In an AC rail traction vehicle, the control strategies are either DTC or FOC, which are highly
responsive precision torque-regulating systems. The level of dynamic braking available in an AC rail traction vehicle is limited by the constant torque and constant-power regions of the drive. If there is sufficient ability to dissipate or store the braking energy on the DC bus bar, there is no significant torque derating relative to the traction case, and full braking force is available down to zero speed.
If braking occurs, the mechanical power is transferred automatically to the DC bus bar of the rail traction vehicle, and energy must be withdrawn from the DC bus bar. This can be dissipated in braking resistors, which are physically similar to the braking resistors in DC rail traction vehicles but controlled with a DC chopper. In an AC-DC-AC rail traction vehicle, the DC bus braking power can be transferred to the overhead power system and potentially used by other trains in the network.
An alternative to the electric dynamic brake is the hydrodynamic brake, which works based on the creation of friction forces arising in fluid flow. Eddy currents are produced within any metal object that has relative motion with a nearby magnetic field, dissipating energy within the metallic object.
Linear railhead eddy current brakes produce a braking force by applying a strong magnetic field to a section of the railhead, while disc brake systems apply the braking field to a rotating brake disc.
The successful application of a rail traction vehicle design requires a comprehensive understanding of its critical parameters in vehicle dynamics studies. Major design characteristics and parameters for rail traction vehicles are classified and discussed. To classify bogies, it is necessary to understand how traction effort is generated and applied to driven axles. Key criteria include the number of wheel pairs enclosed in a vehicle frame, the functioning of the axles in the bogie design, the traction drive design,
and the method of transmission of traction and braking force from the bogie to the locomotive car body.
Rail traction vehicles are commonly equipped with four alternative traction control schemes: locotive or whole vehicle traction control system, bogie traction control, individual wheelset traction control, and independently rotating wheel traction control. These criteria should be systematized and a conceptual scheme on the delivering of traction efforts should be finalized before any traction dynamics study for a rail vehicle starts.
Traction controls in rail vehicles are often limited by creep/wheel slip control principles. Data from angular velocity sensors on wheels or wheelsets is essential for estimating wheel slip. Modern adhesion/traction control systems can use conventional and extended slip control techniques, with the basic operational principle being to maintain the estimated slip value in the stable wheel slip zone.
Conventional systems usually operate with a predefined wheel slip threshold, while extended slip control techniques operate at the peak of the adhesion-slip curve.
Axle loads can be specified in two ways: indicative, where the mass of the rail traction vehicle is carried equally by each axle, and more detailed, determining the variation in load on each individual wheel. For example, a six-axle locomotive can be calculated using the position of the center of mass for the locomotive car body structure. The center of mass is determined by the distance between nominal and sprung masses of the car body and bogies, and the position of the center of mass for the locomotive car body is determined by the axes coordinate system.
Rail traction vehicles (RTVs) use various factors to determine their performance and efficiency. These factors include the rolling radii of left and right wheels, the sprung mass of a bogie, and the unsprung mass of a bogie attributed to each wheel. The maximum tractive effort exerted by a RTV is limited by the adhesion between wheels and rails, and is defined by FTE max t = × µ m g ot ×.
The maximum adhesion/traction coefficient can be achieved by a RTV, with a maximum realized traction coefficient of over 40% only achievable on a dry track with optimal conditions and optimized traction control algorithms. The power output of a RTV is usually defined in watts or horsepower, with the maximum speed being determined by the speed characteristic of the traction electric motor.
The maximum speed can also be further limited by the technical solutions or design of the running gear of a RTV. The maximum traction coefficient can be achieved only on a dry track with ideal conditions and optimized traction control algorithms.
