How about a series? Murder on the Orange Line, Murder on the Blue Line, Murder on the Yellow Line, Murder on the Green Line, and Murder on the Red Line. (Eventually a Murder on the Purple Line, so they have planned).
I like that idea. Set in Virginia, DC, and Maryland. 5 different killers. Or, let's stop and think. Maybe one book, 5 major sections? Hmmm, think, think, think.
At least its in writing, I have something to start with. (and this is my copyrighted material - see the bottom of my page)