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| An arch'" is a structure that spans a space while supporting weight (e.g. a doorway in a stone wall). Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture, but their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures. History. Arches were known by the Mesopotamian, Urartian, Harappan, Egyptian, Babylonian, Greek and Assyrian civilizations, but their use was infrequent and mostly confined to underground structures such as drains where the problem of lateral thrust is greatly diminished. The ancient Romans learned the arch from the Etruscans, refined it and were the first builders to tap its full potential for above ground buildings: "The Romans were the first builders in Europe, perhaps the first in the world, fully to appreciate the advantages of the arch, the vault and the dome." Throughout the Roman empire, their engineers erected arch structures such as bridges, aqueducts, and gates. They also introduced the triumphal arch as a military monument. Vaults began to be used for roofing large interior spaces such as halls and temples, a function which was also assumed by domed structures from the 1st century BC onwards. The Roman arch is semicircular, and built from an odd number of arch bricks (called "voussoirs"). An odd number of bricks is required for there to be a "capstone" or "keystone", the topmost stone in the arch. The Roman arch's shape is the simplest to build, but not the strongest. There is a tendency for the sides to bulge outwards, which must be counteracted by an added weight of masonry to push them inwards. The Romans used this type of semicircular arch freely in many of their secular structures such as aqueducts, palaces and amphitheaters. The semicircular arch was followed in Europe by the pointed Gothic arch or ogive (derived from the Islamic pointed arch in Moorish Spain), whose centreline more closely followed the forces of compression and which was therefore stronger. The semicircular arch can be flattened to make an elliptical arch as in the Ponte Santa Trinita. The parabolic and catenary arches are now known to be the theoretically strongest forms. Parabolic arches were introduced in construction by the Spanish architect Antoni Gaudí, who admired the structural system of Gothic style, but for the buttresses, which he termed “architectural crutches”. The catenary and parabolic arches carry all horizontal thrust to the foundation and so do not need additional elements. The horseshoe arch is based on the semicircular arch, but its lower ends are extended further round the circle until they start to converge. The first examples known are carved into rock in India in the first century AD, while the first known built horseshoe arches are known from Aksum (modern day Ethiopia and Eritrea) from around the 3rd–4th century, around the same time as the earliest contemporary examples in Syria, suggesting either an Aksumite or Syrian origin for the type of arch. It was used in Spanish Visigothic architecture, Islamic architecture and mudéjar architecture, as in the Great Mosque of Damascus and in later Moorish buildings. It was used for decoration rather than for strength. Across the ocean in Mexico and Central America, Mesoamerican civilizations created various types of corbelled arches, such as with the interior tunnels in the Great Pyramid of Cholula and the many styles of corbelled arches built by the Mayan civilization. In Peru, the Inca civilization used a trapezoidal arch in their architecture. The arch is still used today in some modern structures such as bridges. Construction. An arch requires all of its elements to hold it together, raising the question of how an arch is constructed. One answer is to build a frame (historically, of wood) which exactly follows the form of the underside of the arch. This is known as a centre or centring. The voussoirs are laid on it until the arch is complete and self-supporting. For an arch higher than head height, scaffolding would in any case be required by the builders, so the scaffolding can be combined with the arch support. Occasionally arches would fall down when the frame was removed if construction or planning had been incorrect. (The A85 bridge at Dalmally, Scotland suffered this fate on its first attempt, in the 1940s). The interior and lower line or curve of an arch is known as the "intrados". Old arches sometimes need reinforcement due to decay of the keystones, known as bald arch. The gallery shows arch forms displayed in roughly the order in which they were developed. Technical aspects. The arch is significant because, in theory at least, it provides a structure which eliminates tensile stresses in spanning an open space. All the forces are resolved into compressive stresses. This is useful because several of the available building materials such as stone, cast iron and concrete can strongly resist compression but are very weak when tension, shear or torsional stress is applied to them. By using the arch configuration, significant spans can be achieved. This is because all the compressive forces hold it together in a state of equilibrium. This even applies to frictionless surfaces. However, one downside is that an arch pushes outward at the base, and this needs to be restrained in some way, either with heavy sides and friction or angled cuts into bedrock or similar. This same principle holds when the force acting on the arch is not vertical such as in spanning a doorway, but horizontal, such as in arched retaining walls or dams. Even when using concrete, where the structure may be monolithic, the principle of the arch is used so as to benefit from the concrete's strength in resisting compressive stress. Where any other form of stress is raised, it has to be resisted by carefully placed reinforcement rods or fibres. (See Arch bridge.) Other types. The Delicate Arch, a natural arch in Moab, UtahA blind arch is an arch infilled with solid construction so it cannot function as a window, door, or passageway. A dome is a three-dimensional application of the arch, rotated about the center axis. Igloos are notable vernacular structures making use of domes. Natural rock formations may also be referred to as arches. These natural arches are formed by erosion rather than being carved or constructed by man. See Arches National Park for examples. A special form of the arch is the triumphal arch, usually built to celebrate a victory in war. A famous example is the Arc de Triomphe in Paris, France. A vault is an application of the arch extended horizontally in two dimensions; the groin vault is the intersection of two vaults. | A train'" is a connected series of vehicles that move along a track (permanent way) to transport freight or passengers from one place to another. The track usually consists of two rails, but might also be a monorail or maglev guideway. Propulsion for the train is provided by a separate locomotive, or from individual motors in self-propelled multiple units. Most modern trains are powered by diesel locomotives or by electricity supplied by overhead wires or additional rails, although historically (from the early 19th century to the mid-20th century) the steam locomotive was the dominant form of locomotive power. Other sources of power (such as horses, rope or wire, gravity, pneumatics, and gas turbines) are possible. The word 'train' comes from the Old French "trahiner", itself from the Latin "trahere" 'pull, draw'. Types of trains. An electric locomotive -hauled freight train There are various types of train designed for particular purposes. A train can consist of a combination of one or more locomotives and attached railroad cars, or a self-propelled multiple unit (or occasionally a single powered coach, called a railcar). Trains can also be hauled by horses, pulled by a cable, or run downhill by gravity. Special kinds of trains running on corresponding special 'railways' are atmospheric railways, monorails, high-speed railways, maglev, rubber-tired underground, funicular and cog railways. A passenger train may consist of one or several locomotives, and one or more coaches. Alternatively, a train may consist entirely of passenger carrying coaches, some or all of which are powered as a "multiple unit". In many parts of the world, particularly Japan and Europe, high-speed rail is utilized extensively for passenger travel. Freight trains comprise wagons or trucks rather than carriages, though some parcel and mail trains (especially Travelling Post Offices) are outwardly more like passenger trains. Trains can also be 'mixed', comprising both passenger accommodation and freight vehicles. Such mixed trains are most likely to occur where services are infrequent, and running separate passenger and freight trains is not cost-effective, though the differing needs of passengers and freight usually means this is avoided where possible. Special trains are also used for track maintenance; in some places, this is called maintenance of way. In the United Kingdom, a train hauled by two locomotives is said to be "double-headed", and in Canada and the United States it is quite common for a long freight train to be headed by three or more locomotives. A train with a locomotive attached at each end is described as 'top and tailed', this practice typically being used when there are no reversing facilities available. Where a second locomotive is attached temporarily to assist a train up steep banks or grades (or down them by providing braking power) it is referred to as 'banking' in the UK, or 'helper service' in North America. Recently, many loaded trains in the US have been made up with one or more locomotives in the middle or at the rear of the train, operated remotely from the lead cab. This is referred to as "DP" or "Distributed Power." Official terminology. The railway terminology that is used to describe a 'train' varies between countries. In the United Kingdom, the interchangeable terms set'" and "'unit'" are used to refer to a group of permanently or semi-permanently coupled vehicles, such as those of a multiple unit. While when referring to a train made up of a variety of vehicles, or of several sets units, the term "'formation'" is used. (Although the UK public and media often forgo 'formation', for simply 'train'.) The word "'rake'" is also used for a group of coaches or wagons. In the United Kingdom Section 83(1) of the Railways Act 1993 defines "train" as follows: In the United States, the term "'consist'" is used to describe the group of rail vehicles which make up a train. When referring to motive power, "'consist'" refers to the group of locomotives powering the train. Similarly, the term "'trainset'" refers to a group of rolling stock that is permanently or semi-permanently coupled together to form a unified set of equipment (the term is most often applied to passenger train configurations). The Atchison, Topeka and Santa Fe Railway's 1948 operating rules define a train as: "An engine or more than one engine coupled, with or without cars, displaying markers." Motive power. The first trains were rope-hauled, gravity powered or pulled by horses, but from the early 19th century almost all were powered by steam locomotives. From the 1920s onwards they began to be replaced by less labour intensive and cleaner (but more complex and expensive) diesel locomotives and electric locomotives, while at about the same time self-propelled multiple unit vehicles of either power system became much more common in passenger service. In most countries dieselisation of locomotives in day-to-day use was completed by the 1970s. A few countries, most notably the People's Republic of China, where coal and labour are cheap, still use steam locomotives, but this is being gradually phased out. Historic steam trains still run in many other countries, for the leisure and enthusiast market. Electric traction offers a lower cost per mile of train operation but at a higher initial cost, which can only be justified on high traffic lines. Since the cost per mile of construction is much higher, electric traction is less favored on long-distance lines with the exception of long-distance high speed lines. Electric trains receive their current via overhead lines or through a third rail electric system. Passenger trains. A passenger train is one which includes passenger-carrying vehicles. It may be a self-powered multiple unit or railcar, or else a combination of one or more locomotives and one or more unpowered trailers known as coaches, cars or carriages. Passenger trains travel between stations where passengers may join or leave the train. The oversight of the train is the duty of a staff called the conductor. Many of the more prestigious passenger train services have been given a specific name, some of which have become famous in literature and fiction. India has the largest passenger density in the world. Some passenger trains, both long distance and short distanced, may use Bilevel car (double-decker) to hold more passengers per car. Designs and safety of passenger trains has changed dramatically over time. Long-distance trains. Long-distance trains travel between many cities and or regions of a country, and sometimes cross several countries. They often have a dining car or restaurant car to allow passengers to have a meal during the course of their journey. Trains traveling overnight may also have sleeping cars. High-speed trains. Russian Velaro high speed passenger train (a form of multiple unit) One notable and growing long-distance train category in the world is High-speed train. Generally they are faster than 200 km h and often use new separate passenger-only line of high grade standard. Shinkansen in Japan opened in 1964 is the first successful example of newly constructed High-speed train. The fastest train on rails is the French TGV (Train à Grande Vitesse) (French for High Speed Train) which achieved a speed of 574.8 km h (356 mph) in testing in 2007. The fastest commercial speed on rail is currently 350km h of Beijing–Tianjin Intercity Rail in China. TGV runs at a maximum commercial speed of 300-320 km h, as does the German ICE. Generally, High-speed rail is very competitive in less than 3 or 4 hours distance (ex; Tokyo – Osaka in Japan, 500km, 2h 30min, Paris- Lyon in France, 500 km, 2h) in corridor of dense population, but often air has advantage in longer journey. Very fast trains sometimes tilt, like the APT, the Pendolino, or the Talgo. Tilting is a system where the passenger cars automatically lean into curves, reducing the sideways g-forces on passengers and permitting higher speeds on curves in the track with greater passenger comfort. Maglev. In order to achieve much faster operation over 500 km h, innovative Maglev technology has been researched for years. Shanghai Maglev Train, opened in 2003, is the fastest one of 430km h operation. But Maglev has never operated to serve mass inter-city transit so far. Inter-city trains. Trains connecting cities can be distinguished into two groups, inter-city trains, which do not halt at small stations, and trains that serve all stations, usually known as local trains or "stoppers" (and sometimes an intermediate type, usually known as limited-stop). Regional trains. Regional trains usually connect between towns and cities, rather than purely linking major population hubs like inter-city train, and serve local traffic demand in relatively rural area. Commuter trains. For shorter distances many cities have networks of commuter trains, serving the city and its suburbs. Train is very efficient mode of transportation to cope with large traffic demand in metropolis. Compared with road transport, it carries many people with much smaller land area and little air pollution. Some carriages may be laid out to have more standing room than seats, or to facilitate the carrying of prams, cycles or wheelchairs. Some countries have double-decked passenger trains for use in conurbations. Double deck high speed and sleeper trains are becoming more common in mainland Europe. Sometimes extreme congestion of commuter trains becomes a problem. For example, an estimated 3.5 million passengers ride every day on Yamanote Line in Tokyo, Japan, with its 29 stations. For comparison, the New York City Subway carries 4.8 million passengers per day on 26 lines serving 468 stations. To cope with large traffic, special cars in which the bench seats fold up to provide standing room only during the morning rush hour (until 10 a.m.) are operated in Tokyo (E231 series train). This train has as many as six sets of doors on each side to shorten the time for passengers to get on and off at station. Passenger trains usually have emergency brake handles (or a "communication cord") that the public can operate. Misuse is punished by a heavy fine. Rapid transit. Large cities often have a metro system, also called underground, subway or tube. The trains are electrically powered, usually by third rail, and their railroads are separate from other traffic, without level crossings. Usually they run in tunnels in the city center and sometimes on elevated structures in the outer parts of the city. They can accelerate and decelerate faster than heavier, long-distance trains. The term "'rapid transit'" is used for public transport such as commuter trains, metro and light rail. However, in New York City, lines on the New York City Subway have been referred to as "trains". Tram. A light one- or two-car rail vehicle running through the streets is by convention not considered a train but rather a tram, trolley, light-rail vehicle or streetcar, but the distinction is not always strict. In some countries such as the United Kingdom the distinction between a tramway and a railway is precise and defined in law. Light rail. The term light rail is sometimes used for a modern tram, but it may also mean an intermediate form between a tram and a train, similar to metro except that it may have level crossings. These are often protected with crossing gates. They may also be called a trolley. Monorail. Monorail is developed to meet medium-demand traffic in urban transit, buts represent minor technologies in the train field. Named trains. Railway companies often give a name to a train service as a marketing exercise, to raise the profile of the service and hence attract more passengers (and also to gain kudos for the company). Usually, naming is reserved for the most prestigious trains: the high-speed express trains between major cities, stopping at few intermediate stations. The names of services such as the Orient Express, the Flying Scotsman, the Flèche d’Or and the Royal Scot have passed into popular culture. A somewhat less common practice is the naming of freight trains, for the same commercial reasons. The "Condor" was an overnight London-Glasgow express goods train, in the 1960s, hauled by pairs of "Metrovick" diesel locomotives. In the mid-1960s, British Rail introduced the "Freightliner" brand, for the new train services carrying containers between dedicated terminals around the rail network. The Rev. W. Awdry also named freight trains, coining the term "The Flying Kipper" for the overnight express fish train that appeared in his stories in The Railway Series books. Other trains of specific kinds. Heritage trains are operated by volunteers, often railfans, as a tourist attraction. Usually trains are a kind of historic value and retired practical operation. Most of them run weekend and vacation seasons. Airport trains are trains within airport buildings that transport people between terminals. Mine trains are operated in large mine and carry both workers and goods. Freight trains. A freight train (also known as goods train) uses "'freight cars (also known as wagons or trucks) to transport goods or materials (cargo) – essentially any train that is not used for carrying passengers. Much of the world's freight is transported by train, and in the USA the rail system is used more for transporting freight than passengers. Under the right circumstances, transporting freight by train is highly economic, and also more energy efficient than transporting freight by road. Rail freight is most economic when freight is being carried in bulk and over long distances, but is less suited to short distances and small loads. Bulk aggregate movements of a mere twenty miles (32 km) can be cost effective even allowing for trans-shipment costs. These trans-shipment costs dominate in many cases and many modern practices such as container freight are aimed at minimizing these. The main disadvantage of rail freight is its lack of flexibility. For this reason, rail has lost much of the freight business to road competition. Many governments are now trying to encourage more freight onto trains, because of the benefits that it would bring. There are many different types of freight trains, which are used to carry many different kinds of freight, with many different types of wagons. One of the most common types on modern railways are container trains, where containers can be lifted on and off the train by cranes and loaded off or onto trucks or ships. This type of freight train has largely superseded the traditional boxcar (wagon-load) type of freight train, with which the cargo has to be loaded or unloaded manually. In some countries "piggy-back" trains are used: trucks can drive straight onto the train and drive off again when the end destination is reached. A system like this is used through the Channel Tunnel between England and France, and for the trans-Alpine service between France and Italy (this service uses Modalohr road trailer carriers). 'Piggy-back' trains are the fastest growing type of freight trains in the United States, where they are also known as 'trailer on flatcar' or TOFC trains. 'Piggy-back' trains require no special modifications to the vehicles being carried. An alternative type of "inter-modal" vehicle, known as a Roadrailer, is designed to be physically attached to the train. The original trailers were fitted with two sets of wheels: one set flanged, for the trailer to run connected to other such trailers as a rail vehicle in a train; and one set tyred, for use as the semi-trailer of a road vehicle. More modern trailers have only road wheels and are designed to be carried on specially adapted bogies (trucks) when moving on rails. There are also many other types of wagons, such as "low loader" wagons for transporting road vehicles. There are refrigerator cars for transporting foods such as ice cream. There are simple types of open-topped wagons for transporting minerals and bulk material such as coal, and tankers for transporting liquids and gases. Today however most coal and aggregates are moved in hopper wagons that can be filled and discharged rapidly, to enable efficient handling of the materials. Freight trains are sometimes illegally boarded by passengers who do not wish to pay money, or do not have the money to travel by ordinary means. This is referred to as "hopping" and is considered by some communities to be a viable form of transport. Most hoppers sneak into train yards and stow away in boxcars. More bold hoppers will catch a train "on the fly", that is, as it is moving, leading to occasional fatalities. |