Some confusion may be due to my lack of knowledge on the subject. I (perhaps incorrectly) used the "DCC" term to cover all digital systems and "DC" to pertain to analog systems. I'll try to rephrase:
My understanding is that with analog (DC?) three rail, the outer rails are always the same polarity and the inner rail is always opposite in polarity. This functionality facilitated the "toy" segment (whistle, uncoupling, etc.).
Since digital systems function with two rail layouts, does the center rail still offer any functional or feature advantages over two rail?
Hope this makes sense.