The train is definitely your best bet, because traffic (495, BW Parkway, 95) is a toss-up and can be quite problematic even non-rush hour. One accident and you are sitting for hours.

One poster mentioned Amtrak, but you can also use MARC since you're traveling M-F. You'll want to check the schedules because it can be slower (but usually not by much) but it is about 1/3d the price and very frequent during rush hours on weekdays. www.mtamaryland.com

Click on MARC
Click on Penn Line

What you'd do from DCA:

Get on a yellow line train towards Mt. Vernon Square (although I believe they are now testing an extension to Fort Totten so your Metro may say that). Make sure you do not get on a Blue Line train to Largo (they share the same track at DCA).

At Gallery Place/Chinatown switch to a Red Line train headed to Silver Spring or Glenmont. Get on the lead car if you can.

At Union Station get off the train and make a right (as in as soon as you step off the train turn right), this will have you heading towards the escalators on the train side of Union.

Go up the escalators (both sets, one to leave the platform and another to go up into Union Station proper) and walk straight. There will be a bunch of ticket machines you can use to purchase your MARC or Amtrak ticket. Continue straight (there will be a little jog to the right) to the gate area. If you are riding MARC the sign with tracks will be right there, if Amtrak you need ot walk towards the Hudsons News Stand.

Hope this helps!