Road transport system offers higher frictional resistance between the wheels and the road surface, which results into more fuel consumption. In a rail transport system, a stiff wheel rolling on a rigid rail offers lower frictional resistance and requires far less energy per ton-mile as compared to road transport. Therefore, reduction in friction was one of the major reasons for the success of rail transport. Nowadays, rail transport is highly suitable for the fast movement of passengers as well as dense goods. First full scale working railway steam locomotive was built in the United Kingdom in 1804 by Richard Traevithick: the steam locomotive was born. From 1810 onwards, such steam locomotives were in widespread service in coal mines. In 1825, the very first public transport railway was established between Stockton and Darlington in North-East England. In 1879, Siemens displayed the first electrically powered locomotive at the Berlin Commerce Fair. The first fully electrified railway was opened in 1895 in the United States of America: a five-kilometer city tunnel was electrified using a 675 volts overhead system to countering the smoke problem. India's first train ran from Bombay (Mumbai) to Thane for around 34 kilometres on April 16 in the year 1853. The first electric train ran in India with the inauguration of services between Bombay Victoria Terminus and Kurla Harbour on 3rd Feb 1925 for a distance of 9.5 miles and the OHE system was electrified on 1500 V DC. In the year 1957, Indian Railways decided to adopt 25 kV 50 Hz AC electrification system based on French Railway (SNCF) technology.
The London Underground first opened as an "underground railway" in 1863 and its first electrified underground line opened in 1890, making it the world's first metro system. Nowadays the metro system with the longest route length is the Shanghai Metro; the busiest one is the Beijing Subway and the one with the most stations is the New York City Subway. Rapid transit system consists of bus, metro, monorail and light rail systems. The first rapid transit system in India was the Kolkata Metro, which started operations in 1984. The Delhi Metro was India's first modern metro and the third rapid transit system in India overall, after the Kolkata Metro and Chennai Mass Rapid Transit System. Delhi Metro Rail Corporation (DMRC) was incorporated in May 1995, construction started in 1998, and the first section, on the Red Line, opened in 2002. All routes of Delhi Metro are charged with 25kV 50 Hz OHE supply system. There are mainly two types of train coaches in the metro train namely the Trailer Coach and Motor Coach. Two wheels are connected to a straight axle such that both wheels rotate in unison. Complete assembly of wheels and axle is called wheelset. A bogie is a modular subassembly or structure underneath a railway vehicle body (i.e. coach or locomotive) to which wheelsets are attached through bearings and suspension system (i.e. springs). Each motor coach or trailer coach has two bogies and each bogie has two wheelset / axles i.e. total number of axles per coach are four and it is called Bo-Bo arrangement. Some high power locomotives have two bogies and each bogie has three axles i.e. total number of axles per locomotive are six and it is called Co-Co arrangement. Each axle of motor coach is connected to the traction motor through gear system. There are currently several metro systems in India which are either operational or under construction i.e. Kolkata Metro, Chennai Metro, Delhi Metro, Bengaluru Metro, Gurgaon Metro, Mumbai Metro, Jaipur Metro, Kochi Metro, Lucknow Metro, Noida Metro, Hyderabad Metro, Nagpur Metro, Gandhinagar and Ahmedabad Metro and Kanpur Metro. The primary factor on which whether the station has to be underground or elevated depends on the availability of the land, the traffic congestion at the respective route and availability of the funds for construction works. It is to be noted that the cost associated with underground metro station construction is approximate 2.5 to 3 times of an elevated metro station. Metro rail lines in India are composed of both standard gauge and broad gauge and operate at low voltage DC supply as well as high voltage AC supply system. The distance between the inner running faces of the two rails on the same track is called gauge. Broad gauge, standard gauge and narrow gauge (meter gauge) have the spacing of 1676mm, 1435mm and 1000mm respectively. Projects like Delhi Metro used broad gauge for their earliest lines but most new projects in India are on standard gauge as rolling stock imported from Europe is on standard gauge.Back to top
Railway traction system can be classified into mainly three categories:
A steam locomotive is a railway locomotive that produces its pulling power through a steam engine. Steam locomotive consists of fire chamber, boiler, cylinder and gear system, which connect the piston rod to the wheel. Till the development of diesel and electric locomotive, steam locomotives were used widely due to their low initial cost as no track electrification was needed, ample availability of the coal, simplicity of locomotive design, less maintenance, easy speed control and independent operation. These locomotives are fuelled by burning combustible material usually coal, wood, or oil to produce steam in a boiler. The pressurized steam moves reciprocating pistons which are mechanically connected to the locomotive's main wheels (drivers).
Diesel-powered locomotives were built just after World War I. Following the end of World War II, they began to displace steam power on American railroads. By the 1970s, diesel and electric power had replaced steam power on most of the world's railroads though several steam locomotives continue to run on tourist and heritage lines. Similar to any vehicle powered by an internal combustion engine, diesel locomotives require a power transmission system to couple the output of the prime mover to the driving wheels. In the early days of diesel railroad propulsion development; mechanical, hydraulic and electric power transmission systems were employed with varying degrees of success. Of the three, electric transmission has proven the most popular. Nowadays, all modern diesel-powered locomotives have diesel-electric transmission system.
Diesel-electric locomotives could be described as electric locomotives with an on-board generator powered by a diesel engine. Diesel-electric locomotives consist of diesel engine which drives three phase alternator. Output three phase AC supply of alternator is converted into DC supply using solid-state rectifier system. Further, DC supply is fed to the DC series motor through control system consisting of contactors, relay and series resistors. Advanced diesel-electric drives also use three phase traction motor with variable voltage variable frequency (VVVF) control.
Input power supply to the electric locomotive may be either low voltage DC supply or high voltage AC supply. In conventional DC locomotive, low voltage DC supply (i.e. 750 V or 1500V DC) is fed to DC series motors through smoothing reactor, DC switchgear & starting resistance. In conventional AC locomotives used in Indian Railway; 25 kV AC, 50 Hz single phase supply is reduced to low voltage AC single phase supply (approximate 1000 volts) using step down traction transformer. The output of transformer is fed to rectifier block, which convert AC into DC. Further, DC supply is fed to DC series motors through smoothing reactor, DC switchgear & starting resistance. Electric locomotives are ideal for commuter rail service with frequent stops. Electric locomotives have high power output, higher operational speed, smooth control, high efficiency, no air pollution, less noise level and low maintenance as compared to diesel-electric locomotive. Advanced locomotives use three phase traction motor-inverter drive systems with VVVF control. They convert the kinetic energy of the train into the electrical energy during braking mode and the same is fed back to the OHE supply system. The chief disadvantage of electric traction system is the need of high initial expenditure for overhead lines or third rail, substations and control systems.Back to top
Pros and Cons of diesel traction system and electric traction system are described below:
1. The initial cost of the diesel locomotive is higher as compared to electric locomotive for same power rating. However, overall initial cost of electric drive system is much higher than the diesel drive system due to the higher initial cost of catenary, overhead equipment (OHE) and power substations. Therefore, the operation of electric locomotive on high traffic density routes is economic whereas the diesel locomotives are suitable for operation on low traffic density routes.
2. Diesel is an expensive non-renewable fuel source whereas electricity is a cheaper source of energy and can be generated through eco-friendly resources i.e. hydro-electric power plant, nuclear power plant, solar & wind power plant etc. Advanced electric locomotives regenerate electrical energy during braking mode which is fed back to the OHE system, thus making them even more efficient. Therefore, operation of electric locomotive is an environmentally friendly form of rail-transport.
3. Diesel locomotive carries huge volume of fuel and additional propulsion equipment such as diesel engine & main alternator etc. This makes diesel locomotive considerable heavier than the electric locomotive of equivalent power rating. Additional weight of diesel locomotive also increases the fuel consumption during operation.
4. Diesel locomotive has its own small electric power station; however huge power can be supplied to electric locomotive through OHE system. Therefore, the electric locomotives are usually more powerful and reliable than the diesel locomotive.
5. Electric locomotives offer substantially better energy efficiency, lower emissions, less maintenance and lower operating costs as compared to diesel locomotives.
6. The major advantage of the diesel locomotive engine is its ability to work at all territories under all environmental conditions. However, the operation of electric locomotive depends upon the availability of OHE supply. Nowadays, population of diesel locomotives in the world is more than that of the electric locomotives.
Therefore, it can be concluded that both type of locomotives have their own importance and the best policy for rail transportation system is to use electric locomotives as well as diesel locomotives considering preference to electric locomotives operation on heavy traffic density route.Back to top
Main factors which affect the gauge selection are initial cost, load carrying capacity, maximum operating speed, curve radius, geographical conditions of territories and stability of rolling stock etc. Therefore, there is always a trade-off between different pros and cons related to the specific gauge.
Narrow gauge is used in mountainous terrain, where the savings in civil engineering works i.e. bridges, tunnels, cutting of mountains etc. is considerably significant. Narrow gauge also allows the use of sharp curve which is prime requirement of track laying in hilly areas. The initial cost of narrow gauge is also low and suitable for the lightly populated areas having low traffic density, the mining industry and the large-scale construction projects. However, it offers low operating speed and tolerable stability.
Broad gauge allows the operation of train at higher speed with greater stability and suitable for high loading capacity & traffic density. However, broader gauge railways are generally more expensive to build. Due to higher operating speed and more spacing between tracks, the curves of broad gauge have longer radius.
It provides balance between pros and cons of narrow gauge and broad gauge. Approximately 55% of the world's railways use 1435 mm standard gauge.
In addition to the general trade-off, another important factor is standardization. It is easier to adopt an existing standard/popular gauge of rail transport technologies than to invent a new one. The standard gauge is a widely used railway track gauge; therefore, standard gauge equipment and related technology are available in much lower cost in the world market. This also increases the acceptance of standard gauge for new incoming projects.Back to top
Electrification has many advantages but requires significant capital expenditure for development of OHE infrastructure. Selection of supply voltage system depends on economics of energy cost, maintenance cost and initial capital cost as compared to the revenue obtained from the train services. As per European and international standardisation, the most commonly used voltages with either third rail or overhead lines are 600V DC, 750V DC, 1500V DC, 3kV DC, 600V DC, 15 kV AC- 16.7 Hz, 25 kV AC, 50 Hz. High voltage AC supply is converted into low voltage DC supply at train level with the help of transformer and controlled rectifier units.
A third rail is a method of providing electric power to trains, through an additional rail placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system. Third rail systems are always supplied from direct current electricity because the skin effect makes the resistance of steel rails too high with the use of alternating current. Highly magnetic materials have a reduced skin depth owing to their large permeability which results into increase in the resistance effectively at 50Hz supply also. Thus, iron wire is useless for AC power lines except to add mechanical strength by serving as a core to a non-ferromagnetic conductor like aluminium. The trains have metal contact blocks called shoes which make contact with the conductor rail. The traction current is returned to the generating station through the running rails.
Power is voltage times current; so for the same power, train will draw lower current from supply system at higher voltage and vice-versa. Lower current at higher voltage means lower line losses and small cross section area of conductor will be required which results into less expensive conductors; however use of high voltage equipment, switch-gear and insulators increase the initial cost.
Pros and Cons of high voltage AC supply system (25kV AC 50Hz) and low voltage DC system are described below:
1. In high voltage supply system, the value of current to be drawn from OHE system will be low for the given value of power, which results into low line losses, better voltage regulation (low voltage drop in OHE line), higher efficiency and less number of feeder substations (for high voltage AC system, spacing between two feeder substations is about 30-40km as compared to 3-4km for low voltage DC system). Longer spacing between substations makes the high-voltage AC system favourable even for metro services.
2. Huge power can be drawn from high voltage supply system which is prime requirement to achieve higher acceleration of train in the complete speed range. High speed train consumes continuously more power from system; therefore the operation of high speed train is feasible with high voltage AC OHE system.
3. Advanced train has the feature of regenerative braking and returns power to the electrification system so that it may be used elsewhere, by other trains on the same system or returned to the power grid. The feeder substations of DC supply system convert high voltage AC grid supply into low voltage DC supply by using transformer and rectifier modules; however reverse flow of power from DC supply side to AC supply side is not possible with rectifier modules. Sometimes, it happens in DC system that the amount of regenerated energy by trains is more than the energy consumed by remaining trains within the section supplied by one substation. In such situation, the excess regenerated energy cannot be fed back to grid. The feeder substations of AC supply system convert high voltage AC grid supply into 25kV AC supply by using transformer which can flow the power in both direction i.e. grid to train supply system and train supply system to grid. Therefore, with 25kV OHE supply system, the excess regenerated energy can be fed back to grid easily to save the energy.
4. At low voltage DC supply system, train draws huge current from the system for given power. Further increase in the current beyond a certain limit complicates the design of current collection devices and enhances the overall initial cost of system due to increase in number of substations and use of thick conductor size. Therefore, after reaching a power level (approx. 6000 hp), there is no further scope for up-gradation of input power to the train. Therefore, the acceleration and maximum operational speed of the trains operate with DC system cannot be increased beyond a limit. With 25kV OHE supply system; huge power can be transferred to the trains. Therefore, trains operate with 25kV AC OHE supply system can be designed for higher acceleration and operational speed.
5. The cost of underground metro system depends on the size of tunnel. The size of tunnel depends on the adopted gauge and power supply system. Third rail DC supply system is more compact than high voltage AC overhead wires and can be used in smaller-diameter tunnels. Thus, selection of gauge and power supply system indirectly depends on the proposed metro route profile i.e. either it is mainly underground or elevated metro system.
6. With low voltage DC supply system, the motor coaches do not have bulky transformer and rectifier units, which reduce the overall weight of the motor coaches and hence energy consumption. It is to be noted that the overall energy consumption of a train during operation on specific route also increases with increase in the weight of the train.
7. From initial cost point of view, 25 kV AC 50Hz supply system is most economical and energy efficient as compared to low voltage third rail DC traction systems (750V/1500V DC). But from asthetic point of view, appearance of low voltage third rail DC traction system is better because it does not have overhead equipment and conductors.
Therefore, both type of power supply system have their pros and cons and based on them, a detailed analysis is essential before finalization of the power supply system for a proposed metro project.Back to top
High-speed rail is a type of rail transport that operates significantly faster than traditional rail traffic, using an integrated system of specialized rolling stock and dedicated tracks. While there is no single standard that applies worldwide, new lines in excess of 250 km/h and existing lines in excess of 200 km/h are generally considered to be high-speed. The first high speed train began operations in Japan in 1964 and was widely known as the bullet train. The International Union of Railways states that high-speed rail is a set of unique features, not merely a train travelling above a particular speed. Many conventionally hauled trains are able to reach 200 km/h in commercial service but are not considered to be high-speed trains. These include the French SNCF Intercites and German DB IC.
High-speed trains normally operate on standard gauge tracks of continuously welded rail. Many countries like Austria, Belgium, China, France, Germany, Italy, Japan, Poland, Portugal, Russia, South Korea, Spain, Sweden, Taiwan, Turkey, United Kingdom, United States and Uzbekistan have developed high-speed rail to connect major cities.
The criterion of 200 kilometres per hour (120 miles per hour) is selected for several reasons; above this speed, the impacts of geometric defects are intensified, track adhesion is decreased at higher speed, aerodynamic resistance is greatly increased (directly proportional to the square of the speed), pressure fluctuations within tunnels cause passenger discomfort and it becomes difficult for drivers to identify trackside signalling. Train configuration of high speed train is electrical multiple units (EMU) where most of the axles in the trains are driven. Locomotive has limited number of axles and space availability on the axle limit the maximum power rating of the motor. Thus the power of locomotive can not be extened beyond a limits. The power requirement of the high speed train is much higher than that of low speed conventional train of similar length. The number of motorised axles (i.e. power of train) in the EMU can be increased as per the requirement to achieve higher acceleration up to the maximum service speed. The fastest trains in the world are given below:
1. SC Maglev (super conducting magnetic levitation) train in Japan has a top test speed of 603 km/h (375 mph) whereas its maximum operating speed during services is 320 km/h (200 mph).
2. Shanghai Maglev train in China has a top test speed of 501 km/h (311 mph) whereas its maximum operating speed during services is 430 km/h (267 mph).
3. TGV wheeled train in France has a top test speed of 574.8 km/h whereas its maximum operating speed during services is 320 km/h (200 mph).
4. Shinkansen series wheeled train in Japan has a top speed of about 443 km/h whereas its maximum operating speed during service is 320 km/h. It is also known as 'bullet train'.
5. Korea Train eXpress (KTX) in South Korea has a top test speed of 421 km/h whereas its maximum operating speed during services is 305 km/h (190 mph).
6. ICE (Inter City Express) Germany has a top test speed of 363 km/h whereas its maximum operating speed during services is 330 km/h.
7. AVE Spain has a top test speed of 404 km/h whereas its maximum operating speed during services is 310 km/h.
8. Talgo trains come in both locomotive hauled and self-propelled versions. Talgo train tilts naturally inwards on curves to reduce the effects of centrifugal force, which allows it to run faster on curves without causing discomfort to passengers. The maximum operating speed of Talgo 350 train during services is 330 km/h.Back to top