Although MARC and VRE are so similar, they operate totally independently of each other. Riders on one may not even be aware the other exists. This map would help solve that.
The two agencies will probably never merge, but it might someday be possible to integrate their operations to work more like a single system. MARC trains might run across the Potomac into Virginia, and VRE trains might one day continue north into Maryland. It would be difficult but possible.
Comment on this at the version cross-posted to Greater Greater Washington.