Using public transportation how can I travel from Baltimore to Washington DC?

I live near Baltimore and would like to take a day trip to DC without having to drive there. How can I use public transportation to get there quickly and cheaply- any suggestions of where to catch either the MARC train, or subway, light rail- whatever. I've never used any of those before so if someone can... show more I live near Baltimore and would like to take a day trip to DC without having to drive there. How can I use public transportation to get there quickly and cheaply- any suggestions of where to catch either the MARC train, or subway, light rail- whatever. I've never used any of those before so if someone can give me specific routes or lines. I would like to end up as close to the national mall as possible.
Update: I'd also like to know if it's safe to ride on the train. I would be with my boyfriend but still kinda scary
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