Wait the 2nd ave subway will really only be two tracks? Why? How much could it have possibly added to the cost to make it four? Once you're digging the tunnel why no do it right?
Billions of dollars of "studies" and such, I'm sure. NIMBYs hate cut and cover, so we end up digging absurdly expensive deep tunnels that are just worse to use once finished (they take forever to get into, compare deep stations in Washington Heights to Astor Place).