Interesting video about the Northern Virginia Computer Aided Dispatch System produced in 2010. Although interoperability works well in Northern Virginia and Northern Virginia-PG-Montgomery Counties MD, I am not sure if true interoperability (meaning DC on board) has ever been realized.
Fairfax County's CAD Interoperability in the National Capital Region - YouTube
Fairfax County's CAD Interoperability in the National Capital Region - YouTube