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A monorail is a metro or railroad with a track consisting of a single rail（in fact a beam），in contrast to the traditional track with two parallel rails.Monorail vehicles are wider than the beam they run on.
Types and Technical Aspects
There are two major types of monorail systems.In suspended monorails，the train is located under the track，suspended from above.In the more popular straddle-beam monorail，the train straddles the rail，covering it on the sides.The straddle-beam style was popularized by ALWEG.There is also a form of suspended monorail developed by SAFEGE that places the wheels inside the rail.
Modern monorails are powered by electric motors and usually have tires，rather than metal wheels which are found on subway，streetcar（tram），and light rail trains.These wheels roll along the top and sides of the rail to propel and stabilize the train.Most modern monorail systems use switches to move cars between multiple lines or permit two-way travel.Some early monorail systems—noticeably the suspended monorail of Wuppertal（Germany），dating from 1901 and still in operation—have a design that makes it difficult to switch from one line to another.This limitation of the Wuppertal monorail is still mentioned at times in discussions of monorails in spite of the fact for both the suspended and straddle-beam type monorails the problem has been overcome.
Advantages and Disadvantages
Advantages：
The main advantage of monorails over conventional rail systems is that they requireminimal space，both horizontally and vertically.The width required is determined by the monorail vehicle，not the track，and monorail systems are usually elevated，requiring only aminimal footprint for support pillars.
Owing to a smaller footprint they are more attractive than conventional elevated rail lines and visually block only aminimal amount of sky.
They are quieter，since modern monorails use rubber wheels on a concrete track.
Monorails can climb，descend and turn faster than most conventional rail systems.
Monorails are safer than many forms of at-grade transportation.As monorail wraps around its track and therefore cannot derail and unlike a light rail system，there isminimal risk of colliding with traffic or pedestrians.
They cost less to construct and maintain，in particular when compared to underground metro systems.
Disadvantages：
Monorails need their own track.
Although a monorail’s footprint is less than an elevated conventional rail system，it is larger than an underground system’s.
A monorail switch by its very design will leave one track hanging in mid-air at any stated time.Unlike in the case of regular rail switches，coming from this track may cause derailing，with the additional risk of falling several meters to the ground.
Most countries（except Japan）do not have standardized beam specifications for monorails，so most tend to be proprietary systems.
In an emergency，passengers cannot exit at once because the monorail vehicle generally sits on top of its rail and there is no ledge or railing to stand on.They must wait until a fire engine or a cherry picker comes to the rescue.If the monorail vehicle is on fire and rapidly filling with smoke，the passengers may face an unpleasant choice between jumping to the ground（and possibly breaking bones in the process）or staying in the vehicle and risking suffocation.Newer monorail systems resolve this by building emergency walkways alongside the whole track（although this reduces the advantage of visually blocking only aminimal amount of sky）.
There are also some remaining concerns over the speed and capacity of monorails.
New Words and Expressions
beam［bi：m］ n. long thick piece of wood or metal or concrete，etc.，used in construction 梁
suspend［sə′spend］ v. hang up 悬挂或吊起某物
straddle［′strædl］ v. sit or stand across（sth.）with both legs wide apart 跨坐或跨立在（某物）上
tire［′taiə］ （US）=tyre hoop that covers a wheel 轮胎
propel［prəu′pel］ v. move，drive or push（sth.）forward 推进，驱动，推（某物）
stabilize［′steibilaiz］ v. （cause sth./ sb.to）become stable 稳定；变得沉稳
horizontal［′hɔri′zɔntəl］ adj. parallel to the horizon；flat；level 与地平线平行的；平的；水平的
vertical［′və：tikəl］ adj. at a right angle to another line or plane，or to the earth’s surface 垂直的，竖的，直立的
footprint［′futprint］ n. impression of a human or an animal foot on a surface；mark left by a foot 脚印；足迹
collide［kə′laid］ v. ～（with sb./ sth.） （of moving objects or people）strike violently against sth.or each other 猛撞某物或互撞
specification［'spesifi′keiʃən］ n. details and instructions describing the design，materials，etc.of sth.to be made or done 规格；规格说明
proprietary［prəu′praiətəri］ adj. （of goods）manufactured and sold by a particular firm，usu.under patent 独家制造和销售的，专利的
ledge［ledʒ］ n. narrow horizontal shelf coming out from a wall，cliff，etc. 水平的窄长架状突出物；壁架；岩石架
railing［′reiliŋ］ n. fence or barrier made of rails，supported by upright bars 横条的栏杆、栅栏或扶手
suffocate［′sʌfəkeit］ v. （cause sb.to）die as a result of not being able to breathe 窒息而死；闷死
lingering［′liŋɡəriŋ］ adj. remaining 继续存在的
electric motor a motor that converts electricity to mechanical work 电动机
ffire engine special vehicle carrying equipment for fighting large fires 消防车
Notes
1. ALWEG was a transportation company known for pioneering straddle-beam monorails.ALWEG was founded by Swedish industrial magnate Dr.Axel Lennart Wenner-Gren in January 1953 as Alweg-Forschung，GmbH （Alweg Research Corporation），based in Fühlingen，Germany，near Cologne.
2.The SAFEGE monorail system was developed in France.The design was headed by Lucien Chadenson.The design of the system entails a monorail suspended from a box-like track，with an opening in the bottom.The rubber wheels of the train run inside the track.
Unlike previous suspended monorails like the Schwebebahn in Wuppertal，Germany，the tracks are not exposed to inclimate weather，and do not need any cleaning or ice-removal systems.This advantage enables them to run in cities where ice and other conditions would impair the reliablity of the system.
3.A tire （U.S.spelling）or tyre （UK spelling）is a roughly toroidal piece of material placed on the circumference of a wheel，either for the purpose of cushioning or to protect the wheel from wear and tear.
4.A railroad switch （known in British and Australian English as（a set of） points or，in technical usage，a turnout ）is a mechanical installation provided at a point where rail track“A”divides into two tracks，“B”and“C”.Each switch contains a pair of linked tapering rails（point blades）that can be moved laterally into one of two positions，determining whether a train coming from“A”will be led towards“B”or towards“C”.A train coming from“B”or“C”will be led to“A”anyway if the moving parts of the switch are not locked（passage in this direction through a switch is known as a“trailing movement”）.
5. Wuppertal is a city in North Rhine-Westphalia，Germany.It is located on the Wupper river south of the Ruhr area.Population 361，333（2005）.
6.A ffire engine is one of many specialized fire suppression apparatuses.A fire engine is designed to pump water using an engine and onboard water supply，which can be replenished via a fire hydrant，water tender or any other available water source by using suction.Engines are also known as pumpers as they are used to pump water onto fires.Their primary purpose is for direct fire suppression，and may carry many tools including ladders，pike poles，axes，fire extinguishers，and ventilating equipment.Engines are normally staffed with at least three people（a captain，an engineer，and a firefighter，and preferably with a second firefighter），to be able to effectively and safely attack a fire.
Exercises
Ⅰ.Answer the following questions according to the text.
1.What is a monorail？
2.How many types of monorail systems are there？What are they？
3.What do monorails generally have instead of metal wheels？
4.Where are metal wheels found？
5.What advantages does monorail have？
6.Why are monorails safer than many forms of at-grade transportation？
7.Comparing with underground metro systems，do monorails cost more or less？
8.What disadvantages does monorail have？
9.In an emergency，why can’t passengers immediately exit？
10.What happens to the passengers if the monorail vehicle is on fire and rapidly filling with smoke？How do newer monorail systems resolve the problem？
Ⅱ.Choose the best answer to each question.
1.In___________monorails，the train is located under the track，suspended from above.
A.straddle-beam B.ALWEG C.SAFEGE D.suspended
2.Modern monorails generally have___________.
A.tires B.metal wheels C.wooden wheels D.plastic wheels
3.Monorails require___________space.
A.maximum B.minimal C.larger D.medium
4.Newer monorail systems build___________alongside the entire track.
A.emergency walkways B.corridor C.passage D.ledge
5.Monorail vehicles are___________than the beam they run on.
A.narrow B.larger C.smaller D.wider
Ⅲ.Translate the following sentences into Chinese.
1.A monorail is a metro or railroad with a track consisting of a single rail（actually a beam），as opposed to the traditional track with two paralled rails.
2.In the more popular straddle-beam monorail，the train straddles the rail，covering it on the sides.
3.Monorails require their own guideway.
4.Monorails are capable of climbing，descending and turning faster than most conventional rail system.
5.The primary advantage of monorails over conventional rail systems is that they requireminimal space.
Ⅳ.Summary writing.
In no more than 80 words give an account of this passage.Use your own words as far as possible.