Did you know that the WMATA SmartTrip card and the MTA CharmCard are the same card?

That means that you can use SmarTrip cards in Baltimore and CharmCards in the DC region.

From "What is CharmCard":

What is the difference between CharmCard® and the SmarTrip® card?

The SmarTrip® and CharmCard® may look different on the outside, but they are the same smart card on the inside. In fact, you may use either one interchangeably in the Washington, DC and Baltimore metropolitan areas to pay your transit fare and to exit Metro parking lots in the Washington, DC area.

That's actually pretty cool.

The other thing is that if you have a monthly MARC railroad pass, you also get free transfers to MTA subway, bus, and light rail, and WMATA bus base fare, and RideOn bus service in Montgomery County.