railroad engineering[′rāl‚rōd ‚en·jə′nir·iŋ]
A branch of engineering concerned with the design, construction, maintenance, and operation of railways. Railway engineering includes elements of civil, mechanical, industrial, and electrical engineering. It is unique in being concerned with the interaction between moving vehicles (mechanical engineering) and infrastructure (civil engineering). The employment of both a load-supporting guideway and groups or strings of connected vehicles on flanged wheels for the transport of goods and people sets railways apart from other modes of transport. See Civil engineering, Mechanical engineering
The plan view of a railroad track is known as the horizontal alignment. It is made up of a series of curves (arcs of simple circles), tangents (straight tracks), and spirals joining the curves and tangents. Deviations from any of the three are flaws. These imperfections are corrected periodically by a technique known as lining the track.
The side or elevation view of track, composed of a series of straight portions and the vertical curves joining them, is known as the vertical alignment. The vertical change in elevation, in feet, over a horizontal distance of 100 ft is the percent grade. Because the friction coefficient of steel wheels on steel rails is low, railroad grades must also be low, with values from zero to 1.5% fairly common. Two-percent grades are severe, usually requiring helper locomotives. Grades that are more severe, up to about 4%, can be surmounted only with considerable extra operating care and at significant additional expense.
Track gage is the distance between rails. The standard gage throughout the world is 4 ft 8.5 in. Narrow gages of 3 ft and broad gages of 5 and 6 ft have all been tried at various times and in different places.
The function of rail is to guide wheels and distribute their vertical and lateral loads over a wider area. Neither cast nor wrought iron was ideally suited to this task. The development of steel that was three times harder than wrought iron at reasonable cost made it possible for the weight of vehicles and therefore the productivity of railways to increase. Rails are joined end to end by butt welding, whereby continuous rails of over 1000 ft (300 m) in length can be produced. When laid in track, the rails are heavily anchored to restrain movement due to temperature changes. See Steel manufacture
Crossties play important roles in the distribution of wheel loads vertically, longitudinally, and laterally. Each tie must withstand loads up to one-half that imposed on the rail by a wheel. The crosstie must then distribute that load to the ballast surrounding it. Timber crossties vary in section from 6 in. × 6 in. (15 cm × 15 cm) for the lightest applications to 7 in. × 9 in. (18 cm × 23 cm) for heavy-duty track and in length from 8 ft 6 in. (2.6 m) to 9 ft (2.7 m) in length. Well-treated hardwood ties in well-maintained track may be expected to last 30 years or more. Timber crossties become unserviceable after time because of splitting, decay, insect attack, center cracking, mechanical wear, and crushing. Prestressed concrete monoblock crossties are standard in the United Kingdom and parts of continental Europe.
The granular material that supports crossties vertically and restrains them laterally is known as ballast. Ideal ballast is made up of hard, sharp, angular interlocking pieces that drain well and yet permit adjustments to vertical and horizontal alignment. Materials that crush and abrade, creating fines that block drainage or that cement, should not be used. Soft limestones and gravel, including rounded stones, are examples of poor ballast, while crushed granite, trap rock, and hard slags are superior.
The earliest diesel-electric switching locomotives developed about 600 horsepower and road freight units 1350 hp. Single locomotive units of 4000 hp are common. Common practice since dieselization began has been to employ a number of locomotive units coupled together to form a single, more powerful power source.
Locomotive development includes designs using liquefied natural gas as a fuel to reduce environmental pollutants. Locomotives equipped with the necessary power conditioning equipment and squirrel-cage asynchronous motors, made possible by the advent of high-capacity solid-state electronics, exhibit superior adhesion and have no troublesome commutators. They are adept at hauling heavy-tonnage mineral freight, fast passenger trains, or high-speed merchandise trains (freight trains that haul primarily high-value merchandise, as opposed to low-cost raw materials such as coal or grain).
Rail passenger systems such as the Shinkansen in Japan (1964); TGV (Très Grande Vitesse; 1981) in France; ICE (Inter City Eisenbahn; 1991) in Germany; X-2000 in Sweden (1990); or Britain's several High-Speed Intercity Trains are notable for speed and convenience. These systems have developed in context of an awareness of deteriorating highway infrastructures, serious concern with the air pollution generated by automobiles and trucks, increasing traffic congestion in urban areas, and worries over petroleum usage and supply. See Magnetic levitation
High-speed passenger trains require significantly different track configurations (for example, curve superelevation and turnout designs) than do slower freight trains, even those high-valued merchandise or intermodal highway trailers and containers, which frequently travel at speeds of 70 mi/h (110 km/h). These engineering differences are impractical on lines primarily moving heavy mineral freight, where axle loadings and speeds differ even more radically from high-speed passenger trains.