Not for long. The modifications required are minor and can be done by the shed during routine loco maintenance. At the most it will be just a few months before there are enough locos available. For the passengers, diesel or electric traction doesn't make any difference.
And the high rise OHE (and even electrification) is mainly targeted at freight traffic and double stack container traffic. It will certainly help them in reducing the number of freight trains on the tracks and indirectly reduce congestion and better manage operations. It is certainly an efficient idea.
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more... Also if the system were not to be successful, the entire OHE network can be changed back to the standard height with little cost, and the locos can continue working with the high rise pantos only, not requiring any changes.