Joe;
Unfortunately, my scanner frequencies and myself will be separated for the next week or so. But you may find what you are looking for at this site's communications forum. Additionally, railfan.net identifies frequencies. If all else fails, google railroad radio frequencies and you should find what you want.
I can tell you from experience that tracking MM by scanner is tough. They are on the radio relatively infrequently and the terrain is not favorable. I've also heard that they may be using cell phones for at least some of their communications. Depending upon where you are in MD, your better bet might be to catch CSX/MARC/AMTRAC, particularly in central MD and out west in Cumberland. If you are in the Hagerstown area you can also catch NS.
Poppyl