Yes. I agree that it should be done. No worry if today fare difference is not so much between 3rd AC and 3rd AC Economy but tomorrow whenever there will be requirement for hike in fare, they should increase the fare of 3rd, 2nd and 1st AC only.
Fare of 3rd AC Economy can be only slightly more then fare of GR and approximately 20% less then 3rd AC.
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However replacement of sleeper coaches is essential.