“Railways moving bulk goods should be designed with low gradients. High speed passenger lines can have higher grades.”
Typically, railway infrastructure includes fixed physical facilities including the following principal components.
Basic railway infrastructure includes the sub-grade, sub-ballast, ballast, sleepers (also known as crossties), rail, and track fastenings that secure the rail in position relative to the sleepers and to each other. These systems, the foundation for railway infrastructure, should be designed for the proposed purpose of the railway. Railways intended to carry heavy loads will require a solid sub-grade without underlying problems such as soft marshy soils, for example, and a substantial sub-ballast cross section of hard angular rock, typically granite. The ballast section must also be hard angular rock; the rock depth must be sufficient to distribute load stresses throughout the sub-ballast and the rock size must be sufficient to permit rapid water drainage into drainage structures built adjacent to the shoulders of the top ballast section.
Railways take advantage of the very low energy required to roll steel wheels over steel rails. But, because there is little friction between steel wheels and steel rail, railways must have low gradients gentle up and down slopes. As mentioned earlier, railways generally are engineered to have grades of 1.0 to 2.0 percent (10-20 meters per kilometer). Railway designers use many techniques to minimize vertical grades; some are shown above in the cross section diagram. Designers use bridges and tunnels to traverse vertically challenging territory, cuts through rolling hills, and fills in low spots, often with material taken from cuts, to keep tracks as level as possible. They add drainage structures such as culverts concrete pipes or box-like structures that conduct water flows under the tracks and common ditches.
Other terminology commonly found in railway projects is shown below in the schematic of a short railway line:
This plan includes infrastructure component structures maintenance depots, and switches (also called turnouts) and crossovers, which allow trains to change from one track to another, and maintenance and sorting yards, where freight and passenger cars are arranged in the correct order for the outbound train. A device known as a ‘Wye’ is used to turn locomotives, and even whole trains, to face in the opposite direction, replacing turntables that were used in the past.
Single and double track
Many railways are built as single track lines. Trains leave a station or a yard with multiple tracks and move to the next station or yard over a single track. Only one train can operate over single track line at the same time. Single track lines often have sidings at various points where trains moving in opposite directions can meet and pass each other (passing sidings). The capacity of a railway line is determined by the longest time for trains to move between passing sidings. As a rule of thumb, railway engineers estimate capacity in trains per day using “Scott’s Formula” (Figure 2.4) which states:
N = E x 24 x 60 x T
where N = Number of trains/day
E = Efficiency of signaling system (usually between 0.7 and 0.9)
T = longest Travel and stopping time in minutes between passing sidings on a given line
Trains are usually heavy and the same thing that make them energy efficient — low friction losses — make them hard to stop. Each freight car and passenger carriage has air brakes at each wheel to slow and stop trains, but it still takes a lot of distance to stop a train — often a kilometer or more. The higher the speed of the train, and the heavier the train, the longer it takes to bring it to a stop. Similarly, it takes a long time and distance to bring a heavy train out of a passing siding and up to track speed. These factors are taken into account in determining the value of “T” in the equation above. For single track lines with track speeds around 100 kph, with a modern signal system and using passing sidings (passing sidings can hold a typical train) a single track line can typically handle 30 trains a day at most (assuming half are in each direction). As the number of trains increases, interference between trains increases and delays to all trains on the line tend to get larger as well. Railway engineers do many things to increase capacity — increase the speed of trains (this reduces T in the equation), build more sidings (also tending to reduce T), modernize signal systems (increasing E).
As the number of trains increases further, railways will connect passing sidings to provide piece of double track, permitting trains to pass while still moving and saving on the stopping and starting times. Eventually, to create more capacity, the entire line will be double tracked. Capacity can also be an issue with double track lines. Trains can follow each other no closer than the stopping distance for the slowest train; in mixed freight, some trains may be slow — either stopping at many small stations or very heavy, other trains may be fast. Large speed differences between trains tend to limit line capacity even on double track, since trains have to switch tracks to get out of each others way. Some urban rail systems need as many as six tracks to allow the train frequencies needed in dense urban areas.
Signaling and train control
Most busy railways install signals to control train movements; these are akin to road traffic lights and they allow trains to operate in both directions on single or multiple track railways. On a single track signal systems may work only at the siding or station. Modern signal systems have train presence detection and their indications are interlocked with switch positions to prevent trains from moving onto a track if there is oncoming traffic. ‘Automatic block’ is a common signal term for systems that are interlocked with the current siding and with sidings ahead and behind to prevent unsafe train movements.
Advanced signal systems rely on centralized systems to control a large territory. Still more advanced systems have computer controls that help dispatchers make sophisticated decisions about which trains to advance and which to delay. Modern signal systems are computerized train controls that require complex digital communication technology. These systems can enforce control indications and stop trains automatically when they detect unsafe conditions. Pictured at left is part of a modern train control system.
High speed or very busy railways are often electrified; they use electric locomotives and draw electrical power, usually from overhead power distribution systems, but sometimes, in urban railways, via a third rail system at ground level. The diagram below shows components for the electrical distribution system and the wayside signals. Major signal system components include signal boxes, display systems (on some railways, the signal display is inside the locomotive, not along the wayside), and the signal and communications cables needed to control these systems. Electrification system components include masts or poles, and a catenary system that delivers electrical current to the locomotive. In overhead systems, such as the one shown at left, locomotives have a pantograph on top to collect the electrical current. The pantograph slides along the catenary as the train rolls underneath. Several electrification standards are used to power railways; today, the most popular is 25-kVAC for mainline railways but many kilo-meters of 3-kVDC systems, some 15-kVAC systems, and a few 1.5-kVDC systems exist. Many urban railways use 1.5-kVDC electrical power but most now use 750-VDC. Most electrification uses an overhead distribution system like that shown in the diagram, but some use third-rail systems, which are more compact, have smaller urban clearances, and use smaller tunnels; most are 600-750-VDC.
For main line passenger railways, electrification has the advantage of a high power-to-weight ratio—a lot of power (kilowatts or horsepower) available with a relatively light locomotive since locomotives don’t require a diesel engine and generator. This is especially useful if trains need to move fast (faster than say 150 kph, and if a high acceleration rate is needed for station stops and departures. Electrification can be attractive in freight lines, as well, especially those with high volumes (at least 40 million gross-tons per year) and high diesel-fuel prices relative to electricity prices.
Railway electrification is expensive, typically US$3.0-5.0 million/kilometer), including substations. Electrification may also require substantial modification to existing railway signal systems, bridges, and tunnels for the higher clearance required for overhead catenary systems. High initial costs and continuing maintenance costs encourage most commercial railways to carefully consider the implications of electrification. Despite this, about 25 percent of global railway lines are electrified and more than 50 percent of all rail transport is moved by electric traction, according to some reports.
Electric railways can reduce rail transport’s environmental footprint, depending on the electricity source, such as low-emission power plants, and distance to the railway, since up to 30 percent of power plant output can be lost in transmission.
Railway engineers often discuss railway ‘loading gauge’, generally defined as a combination of track gauge, physical clearance envelope, and axle load capacity. Track gauge refers to the distance between the inner surfaces of the rail, illustrated below. Although there are many different rail track gauges in use around the world, the most predominant gauges are the following
Many countries have railway lines built to several different gauges. Why is one gauge selected over another? There are two main reasons—heritage and cost. Many railways were built by foreign engineers who used a gauge that was common in their country of origin. The second reason is cost—narrow gauge is cheaper to build than a broader gauge because cuts and fills are smaller, less earth moving or blasting is required, tunnels can be smaller, and narrower gauges require less ballast and can use smaller, less expensive sleepers. Investors often built narrower gauge railways to keep investment costs down during the early days of railways that were built to exploit natural resources. For example, some Latin American railways built to move banana harvests are only 560 mm, a size that could be built quickly and cheaply and easily relocated.
“Two things are responsible for the gauge of a railway either heritage or cost.”
What are the advantages of various size gauges? Broader track gauges are better for railways that are planned for hauling heavy tonnages; broader gauges provide stability, lower track stresses, and a longer lifespan for track components. During the mid-1980s, Vale (CVRD) built a new 1,000 kilometer broad gauge railway line in the Amazon to move massive quantities of minerals. However, lesser gauges can also effectively haul heavy freight. Vale operates another railway in Brazil, a very fine Cape gauge railway (EFVM) that hauls more than 120 million tons of iron ore concentrate from the mountains in the state of Bello Horizonte to an Atlantic port. This railway serves passengers and general freight customers, too. Cape gauge railways in South Africa efficiently haul millions of tons of coal.
Most of the world’s heavy-haul railways are standard gauge, probably due to the large base of rolling stock and many suppliers of standard gauge components, systems, and associated equipment. Standard gauge appears to be a good compromise between cheaper-to-build narrower gauges and more expensive-to-build broader gauges. Gauge may be an important consideration during design (because of construction costs), but is less important for railway operations once a rail line is built.
“The best gauge is the one that already exists; but new standalone railways can be built to any gauge that makes sense. Standard gauge is a good compromise in most cases.”
A new railway line should match the specifications of the predominant gauge in the region if it is to be part of a national network. However, if a new line is independent of other railway lines and has a specific purpose, gauge choice depends on other design considerations. While there are high-speed passenger services using different gauges, a new railway line for high-speed passenger services would likely be built using standard gauge because most of the specialized rolling stock these railways require are built to (and originally designed for) standard gauge. For example, Spain’s national railway is Iberian gauge—1,668 mm, but Spain used standard gauge when it built high-speed rail lines so the trains could interconnect with French and European lines.
Since most of the world’s railways are standard gauge, there is a wider supply of standard gauge rolling stock, track maintenance and track building machinery. Generally, new lines should be built to standard gauge unless the new line is to be connected to a national network of a different gauge or if there is another compelling reason to select a different gauge.
Clearance envelope or loading gauge
Railway ‘loading gauge’ also refers to the physical clearance envelope (shown in the diagram at left) available for rolling stock. The clearance envelope also determines the size of openings in tunnels and under bridges and the distance from the centerline of the track to station platforms, signs, signal lights, and other trackside devices. Railways with overhead electrification will require more vertical clearance but the maximum size of rolling stock still determines the loading gauge. Generally, the physical clearance envelope takes account of sharp curves and long cars and allows for the swaying or rocking motion of rolling stock. Physical clearance envelope is a critical consideration when railways want to introduce an unusual size of new rolling stock such as bi-level passenger cars or double-stack container equipment that may need clearance envelope expansion.
Axle load—the total permitted weight of a loaded rail wagon or a locomotive divided by the number of axles on the piece of rolling stock—is a critical measure of infrastructure physical capacity and strength. Axle loads are an important element of railway loading gauge and permitted axle loads and the weight of empty freight wagons are key determinants of rail transport efficiency and sustainability.
Many older railways were built to a standard of 16 to 18 tons/axle. India, Russia, and China used 22.5 to 23.5 tons as design limit. Heavy haul railways operate at 32.5 tons/axle (standard in North America with some lines operating at 36 tons/axle); and a new special-purpose heavy-haul railway in Australia has been designed to achieve 40 tons/axle .
The weight of empty freight wagons can affect railway efficiency significantly. Early railway rolling stock design was less precise and metallurgy in steel and castings were of poorer quality, resulting in larger and heavier freight wagon components. However, modern engineering and design systems and high-strength steels and aluminum components now allow much lighter freight wagons with higher capacities.
The figure above shows the best GTK to NTK ratios that could be achieved in the given circumstances. In practice, railways do not average such high ratios because of the normal “Brownian” motion of railway assets — they move the wrong direction, or get redirected, move to be cleaned before the next load, and move to and from repair and inspection facilities. Typical gross to net ratios for freight railways average in the 1.8-1.9 range. Railways with light axle load limits (e.g., 17.5 tons in one of the examples) typically have a GTK to NTK ratio above 2.0. In contrast, the most efficient types of freight are heavy haul and double stack containers. For heavy haul, freight wagon design and high axle loads compensate for returning most freight wagons empty for reloading. Double stack achieves low GTK to NTK ratio due high axle loads, low empty weight, the universality of containers, and the need to return even empty containers. The values for General Rail Freight, with an axle load of 22.5 tons/axle and a 30 percent empty miles rate are typical of Russia, China, and India and in practice average around 2.0. By this measure, light road transport, perhaps for local delivery, is not particularly efficient; but heavy road transport can achieve good efficiency.
Typically, infrastructure strength is measured by track modulus-of the degree of stiffness or resistance to vertical deflection under loads. Higher track modulus values mean greater stiffness, generally higher axle-load capacities, and lower infrastructure wear rates. Track modulus is determined by many factors—gauge, rail weight, sleeper type and spacing, ballast type and thickness, and subgrade quality. Sample values appear in the figure below. Higher values denote greater track stiffness and more stable infrastructure conditions.
Railway reforms and investments that encourage increases in axle loads, acquisition of modern light-weight rolling stock, improvements in rolling stock management and operations, and strengthening of the infrastructure, all work to improve the returns and sustainability of railways.
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