What’s the best way to get from Baltimore to Washington via public transportation?

I’m not too familiar with the trains down in the DC and Baltimore area, so any tips on train lines, fare prices, and anything else useful would be much appreciated!

MARC – Penn Line

One way fare is $7.00 from Baltimore Penn Station to Washington’s Union Station, where you can catch the Red Line of the Washington Metro (subway) to get around.

Click the source link below for the schedule

Limo Washington DC & Virginia & Baltimore Party Bus