As much as I love Amtrak, I wouldn't say that it's a sure thing on time. I did a Fairfax-WIL commute on Monday AMs for about a year and tried train and car. Car was more predictable time wise.
Also, there are no car pool lanes south of B'more. on either 95 or BW Parkway. Listen to traffic or watch google maps to pick the quicker of the two.