After decades of dominance, coal fired locomotives were slowly phased out and replaced with safer and more modern diesel and electric engines. Now, the last mainline steam locomotive has finally been phased out.
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Besides steam- and diesel-powered locomotives, many modern trains operate solely on electrical power. They get the electricity from a third rail, or electrical line, along the track. Transformers transfer the voltage from the lines, and the electrical current drives the motors (AC or DC) on the wheels.
Railway electrification in Great Britain began in the late 19th century. A range of voltages has been used, employing both overhead lines and conductor rails. The two most common systems are 25 kV AC using overhead lines, and the 750 V DC third rail system used in Southeast England and on Merseyrail.
Eventually a softer coal was mined, and by the 1860s and 1870s, coal was accepted as the best fuel for trains. The conversion from wood to coal began in Vermont around 1880 and was complete by 1892, with the bulk of the conversions taking place between 1884 and 1886.
No modern train uses a diesel engine to power their wheels directly. They're used as generators to power electric motors. So the only difference boils down to where the electricity comes from. And even in the most train-friendly countries like France and Japan, huge sections of the network are not electrified.
Similarly, in France, about 45% of the railway network is not electrified and there are more than 1,200 diesel trains. In India, which has the world's third largest railway network, 37% of the trains are pulled by diesel locomotives, Vaishnaw had told Rajya Sabha in December 2021. That's roughly 5,000 trains a day.
It found that passing trains carrying coal add on average 8 micrograms per cubic meter of air (ug/m3) to ambient PM2.5 pollution. That is 2 to 3 ug/m3 more than freight trains contribute.
A train engine requires about a hundred litres of fuel to get it started. So it wouldn't be economical if the engine is stopped and started frequently. This apart, if the engine is stopped, the moving parts' lubrication will also come to a halt.