You can take the marc train to union station from Oriole park right next to m&t. It should cost less than $10 one way
Take the Camden Line, it's right next to the M&T Bank Stadium and Camden Yards
You can take the Marc train from Baltimore to DC. Look at both the Penn and Camden Lines to determine which meets your travel needs
Light rail will take you from the stadium to Penn Station. Actually, easiest way is to get off at Mt.Royal Ave and walk about 2 blocks to Penn Station. From there, the MARC train is $8 to Union Station in DC.
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