If engineering and cost considerations are a limiting factor, bilevel makes a lot more sense for corridors and other heavily traveled routes. In that regard the Siemens purchase is a missed opportunity in my opinion- much, much more capacity could have been added to those routes. There's a variety of bilevel commuter equipment that operates just fine on the NEC, I have difficulty seeing how those designs couldn't have been adapted for intercity use. Standardizing long distance on single-level equipment that can go anywhere in the system makes the most sense.