romikgosai/squad_modified huggingface dataset:
The key advantage of the four-rail system is that neither running rail carries any current. This scheme was introduced because of the problems of return currents, intended to be carried by the earthed (grounded) running rail, flowing through the iron tunnel linings instead. This can cause electrolytic damage and even arcing if the tunnel segments are not electrically bonded together. The problem was exacerbated because the return current also had a tendency to flow through nearby iron pipes forming the water and gas mains. Some of these, particularly Victorian mains that predated London's underground railways, were not constructed to carry currents and had no adequate electrical bonding between pipe segments. The four-rail system solves the problem. Although the supply has an artificially created earth point, this connection is derived by using resistors which ensures that stray earth currents are kept to manageable levels. Power-only rails can be mounted on strongly insulating ceramic chairs to minimise current leak, but this is not possible for running rails which have to be seated on stronger metal chairs to carry the weight of trains. However, elastomeric rubber pads placed between the rails and chairs can now solve part of the problem by insulating the running rails from the current return should there be a leakage through the running rails.

Questions and answers:
['
Q: What danger can return currents cause?
A: electrolytic damage', '
Q: How are stray earth currents kept to a manageable levels? 
A: by using resistors', '
Q: What insures minimal current leak during power rails instalation?
A: ceramic chairs', '
Q: How did return current effect water and gas in iron pipes?
A: water and gas mains', '
Q: What is the crucial advantage of four-rail system?
A: running rail carries any current']
On tracks that London Underground share with National Rail third-rail stock (the Bakerloo and District lines both have such sections), the centre rail is connected to the running rails, allowing both types of train to operate, at a compromise voltage of 660 V. Underground trains pass from one section to the other at speed; lineside electrical connections and resistances separate the two types of supply. These routes were originally solely electrified on the four-rail system by the LNWR before National Rail trains were rewired to their standard three-rail system to simplify rolling stock use.

Questions and answers:
['
Q: Why some sections of Bakerloo and District lines were rewired to three-rail system?
A: to simplify rolling stock use', '
Q: What is the voltage shared two types of trains UK railroad system?
A: 660 V', '
Q: How did it become possible to share the voltage for different types of train?
A: the centre rail is connected to the running rails']
A few lines of the Paris Métro in France operate on a four-rail power scheme because they run on rubber tyres which run on a pair of narrow roadways made of steel and, in some places, concrete. Since the tyres do not conduct the return current, the two guide rails provided outside the running 'roadways' double up as conductor rails, so at least electrically it is a four-rail scheme. One of the guide rails is bonded to the return conventional running rails situated inside the roadway so a single polarity supply is required. The trains are designed to operate from either polarity of supply, because some lines use reversing loops at one end, causing the train to be reversed during every complete journey. The loop was originally provided to save the original steam locomotives having to 'run around' the rest of the train saving much time. Today, the driver does not have to change ends at termini provided with such a loop, but the time saving is not so significant as it takes almost as long to drive round the loop as it does to change ends. Many of the original loops have been lost as lines were extended.

Questions and answers:
['
Q: Why some lines of Paris Metro have to operate on a four-rail system?
A: rubber tyres', '
Q: What was the solution for the return current problem in Paris Metro?
A: two guide rails', '
Q: What is required in order for the guide rails to operate properly?
A: a single polarity supply', '
Q: How train is able to turn around after each completed journey?
A: reversing loops', "
Q: Why was the reversing loop created?
A: having to 'run around' the rest of the train"]
An early advantage of AC is that the power-wasting resistors used in DC locomotives for speed control were not needed in an AC locomotive: multiple taps on the transformer can supply a range of voltages. Separate low-voltage transformer windings supply lighting and the motors driving auxiliary machinery. More recently, the development of very high power semiconductors has caused the classic "universal" AC/DC motor to be largely replaced with the three-phase induction motor fed by a variable frequency drive, a special inverter that varies both frequency and voltage to control motor speed. These drives can run equally well on DC or AC of any frequency, and many modern electric locomotives are designed to handle different supply voltages and frequencies to simplify cross-border operation.

Questions and answers:
['
Q: How can different range of voltages be supplied for AC locomotives?
A: multiple taps on the transformer', '
Q: What was a disadvantage of DC system?
A: power-wasting resistors', '
Q: What taps can provide lighting supply?
A: low-voltage transformer windings', '
Q: What will AC/DC motor be replaced with?
A: three-phase induction motor', '
Q: What is the main advantage of an induction motor?
A: can run equally well on DC or AC of any frequency']
DC commutating electric motors, if fitted with laminated pole pieces, become universal motors because they can also operate on AC; reversing the current in both stator and rotor does not reverse the motor. But the now-standard AC distribution frequencies of 50 and 60 Hz caused difficulties with inductive reactance and eddy current losses. Many railways chose low AC frequencies to overcome these problems. They must be converted from utility power by motor-generators or static inverters at the feeding substations or generated at dedicated traction powerstations.

Questions and answers:
['
Q: How can DC motor turn universal?
A: if fitted with laminated pole pieces', '
Q: What problems did AC distribution cause?
A: inductive reactance and eddy current losses', '
Q: How do railways try to solve the problem of inductive reactance of AC system?
A: low AC frequencies', '
Q: How can low frequencies be obtained?
A: converted from utility power']
High-voltage AC overhead systems are not only for standard gauge national networks. The meter gauge Rhaetian Railway (RhB) and the neighbouring Matterhorn Gotthard Bahn (MGB) operate on 11 kV at 16.7 Hz frequency. Practice has proven that both Swiss and German 15 kV trains can operate under these lower voltages. The RhB started trials of the 11 kV system in 1913 on the Engadin line (St. Moritz-Scuol/Tarasp). The MGB constituents Furka-Oberalp-Bahn (FO) and Brig-Visp-Zermatt Bahn (BVZ) introduced their electric services in 1941 and 1929 respectively, adopting the already proven RhB system.

Questions and answers:
['
Q: How did non-standard gauge trains start to operate with high-voltage AC?
A: adopting the already proven RhB system.', '
Q: When was the first trail of RhB system tested?
A: 1913']
In the United States, 25 Hz, a once-common industrial power frequency is used on Amtrak's 25 Hz traction power system at 12 kV on the Northeast Corridor between Washington, D.C. and New York City and on the Keystone Corridor between Harrisburg, Pennsylvania and Philadelphia. SEPTA's 25 Hz traction power system uses the same 12 kV voltage on the catenary in Northeast Philadelphia. This allows for the trains to operate on both the Amtrak and SEPTA power systems. Apart from having an identical catenary voltage, the power distribution systems of Amtrak and SEPTA are very different. The Amtrak power distribution system has a 138 kV transmission network that provides power to substations which then transform the voltage to 12 kV to feed the catenary system. The SEPTA power distribution system uses a 2:1 ratio autotransformer system, with the catenary fed at 12 kV and a return feeder wire fed at 24 kV. The New York, New Haven and Hartford Railroad used an 11 kV system between New York City and New Haven, Connecticut which was converted to 12.5 kV 60 Hz in 1987.

Questions and answers:
['
Q: What frequency was typically used on US Amtrak?
A: 25 Hz', '
Q: What voltage does SEPTA system use?
A: 12 kV voltage', '
Q: What specification is similar for both Amtrak and Septa systems?
A: catenary voltage', '
Q: When was a 11kV system of NY, New Haven and Hartford was converted to 12.5 kV?
A: 1987']
In the UK, the London, Brighton and South Coast Railway pioneered overhead electrification of its suburban lines in London, London Bridge to Victoria being opened to traffic on 1 December 1909. Victoria to Crystal Palace via Balham and West Norwood opened in May 1911. Peckham Rye to West Norwood opened in June 1912. Further extensions were not made owing to the First World War. Two lines opened in 1925 under the Southern Railway serving Coulsdon North and Sutton railway station. The lines were electrified at 6.7 kV 25 Hz. It was announced in 1926 that all lines were to be converted to DC third rail and the last overhead electric service ran in September 1929.

Questions and answers:
['
Q: When was overhead wires system being used for the first time in UK?
A: 1 December 1909', '
Q: What line used the overhead wire system first Victoria to Crystal Palace or Peckham Rye to West Noorwood?
A: Victoria to Crystal Palace', '
Q: What was the cause of lines not being extended?
A: the First World War', '
Q: What voltage was used in the two lines opened in 1925 of Southern Railway?
A: 6.7 kV 25 Hz']
Three-phase AC railway electrification was used in Italy, Switzerland and the United States in the early twentieth century. Italy was the major user, for lines in the mountainous regions of northern Italy from 1901 until 1976. The first lines were the Burgdorf-Thun line in Switzerland (1899), and the lines of the Ferrovia Alta Valtellina from Colico to Chiavenna and Tirano in Italy, which were electrified in 1901 and 1902. Other lines where the three-phase system were used were the Simplon Tunnel in Switzerland from 1906 to 1930, and the Cascade Tunnel of the Great Northern Railway in the United States from 1909 to 1927.

Questions and answers:
['
Q: What countries  used three-phase AC system in the the beginning of 20th century?
A: Italy, Switzerland and the United States', '
Q: What country was a bigger user compare to the three of them?
A: Italy', '
Q: Where did Italy start using the AC system?
A: in the mountainous regions of northern Italy', '
Q: How long did the AC system last in northern Italy?
A: 1901 until 1976', '
Q: How long did three-phase system operate in Cascade Tunnel?
A: 1909 to 1927']
The first attempts to use standard-frequency single-phase AC were made in Hungary as far back as 1923, by the Hungarian Kálmán Kandó on the line between Budapest-Nyugati and Alag, using 16 kV at 50 Hz. The locomotives carried a four-pole rotating phase converter feeding a single traction motor of the polyphase induction type at 600 to 1,100 V. The number of poles on the 2,500 hp motor could be changed using slip rings to run at one of four synchronous speeds. The tests were a success so, from 1932 until the 1960s, trains on the Budapest-Hegyeshalom line (towards Vienna) regularly used the same system. A few decades after the Second World War, the 16 kV was changed to the Russian and later French 25 kV system.

Questions and answers:
['
Q: What country has first tried to use single-phase AC?
A: Hungary', '
Q: What frequency did the line of Hungarian rail system used in 1923?
A: 16 kV at 50 Hz', '
Q: What type of the converter was used in hungarian locomotives of that time?
A: four-pole rotating phase converter', '
Q: How could the locomotives run on four speed levels?
A: motor could be changed using slip rings', '
Q: What system was adopted in Hungary after WWII?
A: Russian']
To prevent the risk of out-of-phase supplies mixing, sections of line fed from different feeder stations must be kept strictly isolated. This is achieved by Neutral Sections (also known as Phase Breaks), usually provided at feeder stations and midway between them although, typically, only half are in use at any time, the others being provided to allow a feeder station to be shut down and power provided from adjacent feeder stations. Neutral Sections usually consist of an earthed section of wire which is separated from the live wires on either side by insulating material, typically ceramic beads, designed so that the pantograph will smoothly run from one section to the other. The earthed section prevents an arc being drawn from one live section to the other, as the voltage difference may be higher than the normal system voltage if the live sections are on different phases and the protective circuit breakers may not be able to safely interrupt the considerable current that would flow. To prevent the risk of an arc being drawn across from one section of wire to earth, when passing through the neutral section, the train must be coasting and the circuit breakers must be open. In many cases, this is done manually by the drivers. To help them, a warning board is provided just before both the neutral section and an advance warning some distance before. A further board is then provided after the neutral section to tell drivers to re-close the circuit breaker, although drivers must not do this until the rear pantograph has passed this board. In the UK, a system known as Automatic Power Control (APC) automatically opens and closes the circuit breaker, this being achieved by using sets of permanent magnets alongside the track communicating with a detector on the train. The only action needed by the driver is to shut off power and coast and therefore warning boards are still provided at and on the approach to neutral sections.

Questions and answers:
['
Q: What was the main requirement for electric feeder stations?
A: line fed from different feeder stations must be kept strictly isolated', '
Q: What invention prevented lines from getting mixed?
A: Phase Breaks', '
Q: What was the part of wire in Phase Break sections an arc being drawn from one wire to another?
A: The earthed section', '
Q: How were the drivers warned to start coasting the train?
A: warning board', '
Q: What should the driver do in order to open and close the circuit breaker?
A: to shut off power and coast']
Modern electrification systems take AC energy from a power grid which is delivered to a locomotive and converted to a DC voltage to be used by traction motors. These motors may either be DC motors which directly use the DC or they may be 3-phase AC motors which require further conversion of the DC to 3-phase AC (using power electronics). Thus both systems are faced with the same task: converting and transporting high-voltage AC from the power grid to low-voltage DC in the locomotive. Where should this conversion take place and at what voltage and current (AC or DC) should the power flow to the locomotive? And how does all this relate to energy-efficiency? Both the transmission and conversion of electric energy involve losses: ohmic losses in wires and power electronics, magnetic field losses in transformers and smoothing reactors (inductors). Power conversion for a DC system takes place mainly in a railway substation where large, heavy, and more efficient hardware can be used as compared to an AC system where conversion takes place aboard the locomotive where space is limited and losses are significantly higher. Also, the energy used to blow air to cool transformers, power electronics (including rectifiers), and other conversion hardware must be accounted for.

Questions and answers:
['
Q: What nowdays electrification systems can use?
A: DC motors which directly use the DC or they may be 3-phase AC motors', '
Q: What is the main action AC and DC systems have to deal with?
A: converting and transporting', '
Q: What type of losses happen during conversion and transmission in wires and electronics?
A: ohmic losses', '
Q: What kind of losses take place in transformers and inductors during conversion/transmission process?
A: magnetic field losses']
In the Soviet Union, in the 1970s, a comparison was made between systems electrified at 3 kV DC and 25 kV AC (50 Hz). The results showed that percentage losses in the overhead wires (catenary and contact wires) was over 3 times greater for 3 kV DC than for 25 kV AC. But when the conversion losses were all taken into account and added to overhead wire losses (including cooling blower energy) the 25 kV AC lost a somewhat higher percent of energy than for 3 kV DC. Thus in spite of the much higher losses in the catenary, the 3 kV DC was a little more energy efficient than AC in providing energy from the USSR power grid to the terminals of the traction motors (all DC at that time). While both systems use energy in converting higher voltage AC from the USSR's power grid to lower voltage DC, the conversions for the DC system all took place (at higher efficiency) in the railway substation, while most of the conversion for the AC system took place inside the locomotive (at lower efficiency). Consider also that it takes energy to constantly move this mobile conversion hardware over the rails while the stationary hardware in the railway substation doesn't incur this energy cost. For more details see: Wiki: Soviet Union DC vs. AC.

Questions and answers:
['
Q: What two systems were compare in the Soviet Union in 1970?
A: 3 kV DC and 25 kV AC (50 Hz', '
Q: Which system had higher losses in overhead wires?
A: 3 kV DC', '
Q: Which system was used in Soviet Union after all calculations?
A: 3 kV DC', '
Q: What came with lesser lesser cost mobile conversion hardware or stationary hardware?
A: stationary hardware']
Newly electrified lines often show a "sparks effect", whereby electrification in passenger rail systems leads to significant jumps in patronage / revenue. The reasons may include electric trains being seen as more modern and attractive to ride, faster and smoother service, and the fact that electrification often goes hand in hand with a general infrastructure and rolling stock overhaul / replacement, which leads to better service quality (in a way that theoretically could also be achieved by doing similar upgrades yet without electrification). Whatever the causes of the sparks effect, it is well established for numerous routes that have electrified over decades.

Questions and answers:
['
Q: What can be seen in the newly electrified lines?
A: "sparks effect"', '
Q: What can electrification of modern trains effect? 
A: patronage / revenue', '
Q: How can better service quality be achieved?
A: rolling stock overhaul / replacement']
Network effects are a large factor with electrification. When converting lines to electric, the connections with other lines must be considered. Some electrifications have subsequently been removed because of the through traffic to non-electrified lines. If through traffic is to have any benefit, time consuming engine switches must occur to make such connections or expensive dual mode engines must be used. This is mostly an issue for long distance trips, but many lines come to be dominated by through traffic from long-haul freight trains (usually running coal, ore, or containers to or from ports). In theory, these trains could enjoy dramatic savings through electrification, but it can be too costly to extend electrification to isolated areas, and unless an entire network is electrified, companies often find that they need to continue use of diesel trains even if sections are electrified. The increasing demand for container traffic which is more efficient when utilizing the double-stack car also has network effect issues with existing electrifications due to insufficient clearance of overhead electrical lines for these trains, but electrification can be built or modified to have sufficient clearance, at additional cost.

Questions and answers:
['
Q: What is a major factor whn it comes to electrification?
A: Network effects', '
Q: What was the reason some electrifications were removed after a while?
A: through traffic to non-electrified lines', '
Q: What can be a drawback of electrification of long distance freight trains?
A: electrification to isolated areas', '
Q: Where can the issue of through traffic benefits occur?
A: long distance trips', '
Q: What does the increasing demand for container traffic make companies use more often?
A: diesel trains']
Additionally, there are issues of connections between different electrical services, particularly connecting intercity lines with sections electrified for commuter traffic, but also between commuter lines built to different standards. This can cause electrification of certain connections to be very expensive simply because of the implications on the sections it is connecting. Many lines have come to be overlaid with multiple electrification standards for different trains to avoid having to replace the existing rolling stock on those lines. Obviously, this requires that the economics of a particular connection must be more compelling and this has prevented complete electrification of many lines. In a few cases, there are diesel trains running along completely electrified routes and this can be due to incompatibility of electrification standards along the route.

Questions and answers:
['
Q: What is the other issue that comes to sight when using electrification system?
A: connections between different electrical services', '
Q: Why the commuter lines built to different standards can cause be complicated?
A: the implications on the sections it is connecting', '
Q: What is the solution many lines came up with in order to avoid replacing present rolling stock?
A: to be overlaid with multiple electrification standards', '
Q: Why are disel trains  still used on electrified routes?
A: due to incompatibility of electrification standards']
Central station electricity can often be generated with higher efficiency than a mobile engine/generator. While the efficiency of power plant generation and diesel locomotive generation are roughly the same in the nominal regime, diesel motors decrease in efficiency in non-nominal regimes at low power  while if an electric power plant needs to generate less power it will shut down its least efficient generators, thereby increasing efficiency. The electric train can save energy (as compared to diesel) by regenerative braking and by not needing to consume energy by idling as diesel locomotives do when stopped or coasting. However, electric rolling stock may run cooling blowers when stopped or coasting, thus consuming energy.

Questions and answers:
['
Q: Which of two can be more efficiently electricified?
A: Central station', '
Q: How can electric power plant become more power efficient?
A: it will shut down its least efficient generators', '
Q: How can electric train be more energy efficient?
A: by regenerative braking', '
Q: Which type of train continues to use energy while coasting or being stopped?
A: diesel', '
Q: What causes electric trains to waste energy?
A: cooling blowers']