that's since non AC travel in IR is too subsidized. IR staff costs have increased after Newer pay commissions like 5th pay, 6th pay, etc while ticket prices have increased but not in proportion to increase in costs.

IR doesn't want to do a significant increase in non AC tickets, so it is encouraging passengers to switch to AC economy from SL class.
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If IR wants poor people to shift, it must CAP AC economy fares to ONLY 2 X times the sleeper fare. I don't know if it will happen.

* Most probably the discount may be 0-15% i.e. it may cost about 2.5-3 times SL.
* IR is working on the plan to operate trains at 160kmph on Delhi-Mumbai, Delhi-Howrah routes after completion of the ongoing Western & Eastern DFC projects. These shall be fully ac trains.
* I would like to see all the express trains operating at 160kmph on these routes with the SL coaches being replaced by ACIII economy coaches.
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But for that to happen, fare of ACIII coaches should be 1.5-2 times the SL fare just like buses. With a very high avg speed of of these trains (80-100 kmph), people may be ready to pay extra for time saved as well as for comfort of the new ACIII-E coaches.