There haven't been many lines that have been converted from DC to AC, but one was the North London Line, which had 3rd rail extended in the mid 1980s through to North Woolwich, and then much of the line converted to 25Kv about 10 years later. The actual costs and man-hours of the two sets of works must be still around, and it would be interesting to see them compared.
I recall actually seeing the works train laying the 3rd rail from Canning Town to North Woolwich, half double and half single track. I think it took them two weekends to lay it, the insulator pots having been previously fixed. No super "High Output" train, no super-Swiss project design offices, no big civils for bridge raising or whatever. Just one works train propelled by a Class 08. It wasn't even as if the Chesterton Junction, Cambridge, PW crew, who did the work, were that familiar with 3rd rail. In comparison to putting in 25Kv masts and wires, the time, effort and cost was a pittance.
