Third Rail is difficult to maintain and expensive too.
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Out of all metros, only kol metro and namma metro have it...

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All of the Delhi Metro's rolling stock is designed for 25KV AC overhead lines and their automatic signalling systems also have circuits designed for an AC OHE system. It will not be possible to switch to a third rail system without spending almost as much amount as was spent on constructing the line.

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Actually, third rail is extremely cheap and if the routes are elevated with nothing trespassing on the route, then it is extremely easy and cheap to maintain also.

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Ohh.
But I have heard and read entirely opposite scenario.

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I Think its least expensive as there is no natural interference on the electric supply wires. And At least should be opted for new lines.

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