Oh no; it does in Marion county as well! (Indianapolis)
Either that or even BETTER: Light Rail!!!
This boondoggle WILL occur; because certain people want it.
I think expanding MARC service could help with congestion. Unfortunately to expand service enough to even pretend to have an impact would cost billions and be a traffic impediment while it is being constructed. Close to tripling the number of trains on the Brunswick line would likely require at a minimum placing sidings for most of the busier stations if not triple tracking the entire main line between Union station and Point of Rocks.
There are several choke points in Montgomery county that road improvements might help, however, very few of them have the potential to be panaceas, and all of them would only either be temporary in their congestion relief or cause the congestion producing bottle neck to simply be shifted to the next choke point.
As for light rail, I am not sure of the point of the Purple line, as I do not think it actually brings enough people from where they are to where they want to go. Most of the heavy congestion in the DC area is people using the radial arteries to travel in or out of DC and the inner suburbs, the Purple line will parallel the Beltway which is used to travel between one radial and the other.