how can I get to DC from Baltimore airport at night ? I will arrive at Baltimore 11:30 and I want to go to DC?

Airport cabs operate 24 hours per day. The airport will still be plenty busy at 11:30 at night. A cab from BWI to DC downtown will run you about $75. They take credit card.

Check the Amtrak schedule.

On weeknights, Amtrak trains stop at BWI at approximately 11:23, 12:23 and 1:30am on their way to Washington Union Station. There's a free shuttle bus from BWI to the Amtrak station.

Here's a link to the Amtrak website. You'll be going from "BWI" to "WAS".
http://www.amtrak.com

Note that the time of the last train varies by the day of the week.

Rt 195 to 695 to 95 to dc