One more thing cost impact due to delay not matters more as Railways enter agreement after finalizing the Capital amount with discount and negotiations with contractor and also railways can take good discount as double line to be electrified which makes more RKM electrification..But When u get one line Electrified by one contractor and everything like substations Scada Controlling he will design for Single line with provisions for future double line but the exact Testing commissionin he will do for Single line as other line will be out of his scope of contract and for that other line railways need to call New Tender and again decide lowest bid or any contractor to award the project and u can't claim more discount and Also cost will be sure more due to cost impact of delay of market rate for the items going to be used.And also new contractor adds his overheads...
more... makes cost of project more compared to single contractor who put his overheads for both lines RE. So now u say One contractor for both line with single overhead cost is profitable or Two contractors separately wit their separate company overheads add on on project cost is profitable ? I agree if delay for RE until doubling is complete makes more cost impact but that can be suppressed with contractor giving more RKM for RE and taking good Discount and also getting single company overhead on project cost which in turn makes no big deal on cost impact increase due to delay..