Rail Track Design for Line Speed Improvement - Research Paper Example

Rail Track Design for Line Speed Improvement Executive summary Rail transport is increasingly becoming important in each and every passing day. The reason being that it eases the congestion in major towns and cities. Due to the ever increasing population, urbanization and demand for transport of goods and services, the demand for rail transport is on the increase. This calls for trains which are fast and able to carry heavy loads. In line with these developments, the rails must be redesigned to carry heavy loads and guide the fast moving trains. These papers looks at different methods of redesigning the rail tracks to be able to resist increased load due to cargo weight and improve stability of the fast moving trains. The design issue under consideration includes route surveying, track component identification, designing the rail curve and other design aspects that help in improving rail transport. Suggestion indicated can be implemented by any train organization seeking to improve or optimize there services (Mundrey, 1993). Introduction Rail tracks are used for the guidance of trains, and consist of two steel rails laid in parallel. The rails guide the train in motion without the need for steering. The rails are laid on sleepers. The sleeper are also referred to as the cross ties. The sleepers are embedded in the ballast and form the rail road track. The rails are fastened to the sleeper by the use of spikes, lag screws, bolts clips and pandrol clips. The type of fastening sleepers depends on the type of sleepers. For the concrete sleepers clips are used, for the wooden sleepers’ spikes are used and for steel sleepers, bolts are commonly used. Rail design The rail design encompasses the following major issues: Initial design: Convectional rail design involved route surveying and finding the most economical route for the rail line. The engineer worked on ways of designing the rail so as to surpass the geographical obstacles (Engineering Policy Group (EPG). 2009). Currently rail design encompasses many design aspects. These design aspect results from the need for greater speeds and heavy loads. Due to these factors, the most important consideration for the rail design are: Route surveying. This involves planning the rail routes so that it passes through the most economical route (Hickerson, 1967). Long term traffic levels; if the projection shows that in the long run the traffic will increase. Design consideration for expansion of the rail as well as incorporating trains with high speed and heavy should be taken into consideration. Environmental concern: the design should focus on an environmental friendly rail that does not damage the environment. Politics: Government influences and funding greatly affect the quality of the rail and also its long term benefits. Land issues; when designing for increased speed, huge curves are required; the constraining factor to this is the limitation of land. Economic factors: this determines the type of rail constructed. Well funded projects can archive most of the design requirements. Design of The track bed The rails are laid on a bed of stones, ballast is commonly used. The bed is comprised of the following major layers; The subsoil: this is the natural ground in which the rail is to be laid. The sub grade: this comprises of compacted soil. The compaction and removal of clay from this layer prevents the expansion and compression preventing the sinking of the rail which damages this track. The soils used in this layer must not expand or compress due to water absorption. Compression of the soil also helps is preventing the infiltration of water and clay. Blanket: this is a layer of stone dust or sand, it is mixed with impervious plastic. The layer prevents the upward infiltration of clay and water which may destroy the rails. Ballast: This is a layer of stone. The type of stone used and the grade of the ballast determine its load bearing characteristics. Sleepers: it is also called the tie plate. It distributes the load of the rail over a large area They support the rail tracks in the right position. The rail track: They support the train and guide it. They are made of steel. They distribute the load to the sleepers. Improving the Track Bed for Line Speed Improvement Some of the modification for bed includes greater compression of the subsoil and sub grade to enable the track to withstand compressive force due to increased train weight and cargo. Blanket The blanket layer should also be made of materials that can withstand the forces and also resists any water infiltration. The material used for the ballast The ballasting materials and grade of the ballast materials are important. Selection of the right grade enables the permanent way to withstand the heavy compressive forces of the train and cargo. Granite rocks which are hard are the best to use. Increased load will mean that the ballast layer be increased also. This helps in improving the load bearing capacity of the ballast. Thus when designing for increased rail speed and load, the following are necessary The increase of the depth of the ballast layer. This increases the load bearing capacity of the ballast and boosts stability. The increase of the cross-sectional area of the ballast. This results from effects of increasing the size of the sleepers and rail gauge. Rail alignment design consideration When designing the rails, one of the most important considerations is the alignment requirement, rails guide the train which means that the steering is not controlled by the operator. The operator only controls the longitudinal aspect (speed, forward and reverse motion control) but has no control over the horizontal aspects. Another aspect is that the vehicular unit is long and thin. For correct alignment in design, especially for high speed and increased load, the designer has to factor in the following; The train traffic (passenger rail, freight rail, light rail and rail length) Volume of the traffic Speed of the rail vehicular unit When designing for the speed and heavy loads the following aspects of alignment must be considered. These aspects include The tangents The grades The horizontal curves The vertical curves The spirals The super elevations Design of the tangents and rail gauges A tangent is the straight rail track. Their major influence on the vehicular track is its length and the gauge. Different rail have different gauges. The gauge can be defined as the distance between two inner surfaces of the two parallel rail lines. There are a set of standard rail gauges used in the world. These gauges include; The standard gauge (1435 mm) (Federal Railway Administration. 1982) The broad gauge The narrow gauge The dual gauge The gauges tolerances define how these gauges differ form the standard gauges and still be refereed to fall in any of the above stated gauges. Gauge consideration when designing for high speed and greater load When designing for greater speed and load, the gauge consideration is of prime importance; broader gauges allow for the design of a vehicular unit that are wide and hence are able to support a greater load. Also, broader gauges means that the sleeper size is much bigger, this allow for greater distribution of the load. Broader gauges will also permit increase the stability of the train. Wider objects are more stable as compared to slim tall vehicular units. Broad gauges are especially used where huge loads are to be transported and also for the easy transfer of the rolling stock. Broad gauges are however expensive to construct. Another solution is to use the dual gauges which allow several gauges to be used on one track, this saves the costs but introduces complexities in signaling and maintenance operations in the rail track. To cut back on cost, different gauges can be incorporated in one rail track at different sections. In mountainous areas, standard gauges can be used. In the flat areas and where the traffic is very high, broad gauge can be used. The narrow gauge is completely unsuitable for high speed and heavy axial load system (Simmons and Biddle, 1997). Loading gauge The length of the tangent track is of important consideration, for rails subjected to high traffic and heavy loads, the rail track gains heat due to increased friction. The positioning of the rail gaps is important; these gaps should be placed at closer position than normal speed train to allow for increased expansion. Welded rail joints can be used as they reduce the maintenance cost associated with rail tracks with many gaps. In fact when designing for high speed trains welded rails are the most economical as they give smooth ride and less friction. When designing for high speed and heavy load, the straightness of the track is very important; the designer should work to reduce the curved sections and change of grade as much as possible. Straight paths allow greater speeds as the train is not subjected to centripetal and centrifugal forces that act or the vehicular unit negotiating a corner. Tangent tracks are also cheap to build but are sometimes impossible to build due to land restriction and geographical obstacles, these results to some sections of the rail having curved sections. The change of grade The grade is the slope of the rail road track. This is common in hilly and mountainous areas. When the train is acceding or descending an inclined surface there is a change of grade. It is expressed as a percentage rise for a given length covered. The diagram below illustrates the change of grade and parameters for calculating it. Figure 1 showing grade The maximum allowable grade is about 2 %, if the grade is too high, the trains experiences problems when climbing and may skid when descending resulting to an accident. For high speed trains, the grade should be reduced as much as possible, a sharp change in slope subject the passages to a lot of discomfort particularly when the train is moving at high speed. Thus, to improve the ergonomics it is necessary to reduce the grade. Another problem is that when high speed trains descend the total velocity increases due to gravitational pull. This may derail the train or cause excessive braking and skidding. When designing for heavy loads, the grade should be kept as low as possible. This is because of the forces that pull the train backwards Curved track grade The effect of a curved section along a grade is to increase the effective slope of the grade. The sharper the bend, the greater the effective grade, thus when designing for increased speed, sharp bends along the grade must be minimized (China Ministry of Railways. 1997). The horizontal curves These are curved sections along the rail track, at such points the straight section of the rail track (tangent track) meets a curved rail. There must be a proper transition from the tangent to the curved rail track. This is mathematically computed using the Euler spiral fitted between the tangent and circular curve. The Euler spiral starts as an infinite curve and ends as a curve with a specific radius. The design is necessary to prevent abrupt changes resulting to passenger discomfort, abrupt forces and centripetal acceleration that may cause derailment and overturning of the train. The resultant is a broad curve. The curve is also raised to a given degree. This is known as super elevation. The centrifugal forces are a product of the vehicular speed and weight. Hence, when designing for heavy loads and high speed, the maximum practical radius should be considered. The larger the curve radius, the greater the speed. The constraining condition is the availability of land and other geographical obstacles. The curve starts at zero, corresponding to the straight section and increases linearly to end at the curvature of the horizontal circular section. The radius of the curvature is given by the reciprocal of the radius of the curve. (Kellogg, 1997). Figure 2 showing the transition and tangent track The vertical curve The vertical curve represents the superelevation. Superelevation is used to counter the effects of centrifugal force, the magnitude of the centrifugal force is determined by the speed of the train and it’s mass. Based on the computed calculation an equilibrium elevation can be archived, however, this is not normally applied because different trains moving at differing speed using the same rail will result to different superelevation curves. Trains rarely overturn but they derail before overturning. For the stability of the train, the resultant forces must fall between the middle third of the track. This falls between the middle 20 inches for the standard rails. Figure 3 showing the effects of centrifugal force The equation for calculating the centrifugal force is Where g is the acceleration due to gravity W is the weight in pounds V is the velocity in feet per second r is the radius of curvature F is the centrifugal force From the above equation, it can be verified that the centrifugal force increases with the square of speed and is inversely proportional to the radii of curvature. Figure 4 showing the train at equilibrium the correct weight and speed Figure 5. Overbalanced condition of the train due to low speed and weight. Figure 6 showing the under balanced train condition due to high speed From the three diagrams that is figure 4 to figure 6, the maximum allowable speed is equal to Where D is the degree of curvature 3 is the amount of under balance Ea average elevation of the rail Vmax is the maximum allowable speed When the resultant force falls outside the two rails, the train will tip and eventually overturn. The factors that are mostly considered when establishing this curve are the established limits. The overturning speed varies and also depends on the center of gravity of the train. A limit is set for the elevation that is usually allowed. The picture below shows different trains and the balance conditions. The maximum speed is computed as a function of the elevation and the degree of curvature (Meunier, 1983). To archive high speeds, super elevation curve must be designed based on the speeds of trains operating on that rail. One common phenomenon is to have trains moving at optimum speeds. This helps in designing superelevation curves. This method has been applied by TGV trains. In their design, one line is reserved for TGV super trains. This allows them to move extremely fast and to have a higher superelevation curve as compared to the other rail tracks. It is also possible to incorporate stepper grades for TGV rails. Different mathematical spirals can be used to determine the superelevation curves these curves include Euler, Clothoid, Searles, Talbot and AREMA. when designing superelevation curves for high speeds the following factors are considered. Increasing the horizontal curvature of the curve Increasing the effective length of the super elevation curve Standardizing the speeds of the trains using the high speed track, this helps in increasing the superelevation length. Using a broad gauge. The broad gauge ensures that the train cannot trip over due to the centrifugal forces (Sweedler, 1999). Rail profile design for high speed The major components are; The rail profile The rail profile comprises of a universal beam or an ‘I’ beam which is asymmetrical. The train wheels run on these rails. They are usually subjected to high stresses and must be made from high quality steel alloys. The load bearing characteristics of the rails depends upon the structural size of the rail. Heavier rails are able to carry heavily loaded trains. Rails are usually graded in terms of the weight over a given standard length. Based on this classification, rails can be classified as heavy rail and light rail. Heavy rail withstand greater axial loads and high speeds without failing (Firuziaan and Estorff, 2002). Different rail parameters can be adjusted to enable the rail operate at increased speeds and be able to withstand heavier loads, these include Development of superior alloy materials which can withstand greater axial loading and compressive forces. Optimizing the profile shape of the asymmetrical I beam used in construction of the railway. Use of heavier ‘I’ beam to enable the rail to support heavier trains moving at increased speed. Use of different rail profiles. These profiles include Bullhead rail Grooved rail Vignoles or flat bottomed rails Flanged rails Bridge rails or inverted U shaped rail profile The sleepers The sleeper or railroad tie is a rectangular shaped metal which supports the rails in position. The rails are also fixed on the sleeper. The main function of the sleeper is to Transfer the axial load to the ballast and to secure the rails at the right position and maintain the correct gauge. (AREMA, 1998). There are three main types of sleepers used in rail transport. These are; The wooden sleeper The steel sleeper The concrete sleeper The design of the sleeper can be modified to accommodate greater loads by: Using materials especially steel alloys that can resists compressive forces Using sleeper with a greater cross-sectional area Concrete and wooden sleeper would be unsuitable for high speed trains Using more heavy steel to manufacture the sleepers Check rails Check rails are fitted inside the rail line to guide the train when negotiating a curve. Check rails can be used on trains moving at high speeds and carrying heavy loads. Fisher plate replacement The fisher plate is used in the rail design to join two rails separated by a gap. This gap is intentionally left for expansion of steel. When designing for high speed it is necessary to replace the fisher plate with welded joint which reduces friction and allows smoother running of the rail wheel resulting to increased speeds (Thomas, 2002). Track maintenance for increased speeds When designing for high speeds, Periodic checking of the rails is necessary as they are subjected to heavy stresses. Some of the maintenance aspects considered when designing high speed rails are; Use welded rails to reduce the frequent maintenance operations Technology to detect and locate any rail cracks or breaks and relay information to a control center Other design consideration Other design consideration in improving rail line speeds are: Research on an accurate method of measuring the rail stresses and also monitors the changes of stress with time in order to design a better fast rail maintenance procedure especially for the welded rails. Improving the structural properties of different rail elements such as the frogs, switch points, guard rails, fastenings that can withstand heavy loads, and improving the stock rails. Concrete slab testing to ensure that they perform without fail in heavy axle and high speed rail systems Improving the turnout geometry Testing longitudinal rail stress on very long bridges and viaducts to ensure proper functionality under heavy axial loading from the trains References Mundrey, J. 1993. Railway Track Engineering, 2nd Ed. New Delhi, India: Tata McGraw-Hill Publishing Company Limited. Federal Railway Administration. 1982. Track Safety Standards, 49 Code of Federal Regulations Part 213.Washington, D.C.: U.S. Department of Transportation. China Ministry of Railways. 1997. Present Situation of Chinese Railways and Upgrading Program - Final Report. Beijing, China. Internal report prepared for World Bank Fifth Railway Project. American Railway Engineering and Maintenance of Way Association (AREMA). 1998. Manual for Railway Engineering. Washington, D.C. Sweedler, B. 1999. Toward a Safer Future: National Transportation Safety Board Priorities.TR News, No. 201, March-April 1999. Meunier, J.1983. On The Fast Track: French Railway Modernization and the Origins of the TGV. Associated Press 2007. "French train breaks speed record” cnn.com. CNN. Available at http://web.archive.org/web/20070407194558/http://www.cnn.com/2007/WORLD/europe/04/03/TGVspeedrecord.ap/index.html . Accessed July 10 2009 Thomas, P. 2002. Railroad Track Structure System Design. Sn Firuziaan, M. and Estorff, O. 2002. Simulation of the Dynamic Behavior of Bedding-Foundation-Soil in the Time Domain. New York : Springer Verlag. Engineering Policy Group (EPG). 2009. “Route Surveying.” Engineering Policy Guide. Available at http://epg.modot.org/index.php?title=238.3_Route_Surveying. Accessed July 7 2009. Biddle, Gordon (1990). The Railway Surveyors. Chertsey, UK: Ian Allen. Hickerson, T.1967. Route Location and Design. New York: McGraw Hill. Kellogg, N. 1997. The Transition Curve or Curve of Adjustment (3rd edition ed.). New York: McGraw. http://books.google.com/books?id=ZVZCzW2codgC. Simmons, J and Biddle, G.1997. The Oxford Companion to British Railway History. Oxford: Oxford University Press.  Read More