A friend of mine who commutes in the New York to DC corridor has used Amtrak a few times. Buy a ticket, travel in uniform, and (as a previous poster mentioned) more often than not, the conductor won't lift your ticket and you can get it refunded afterward. He said he's running about 50/50 so far, but knows guys who get the free ride up to 80% of the time.

I wouldn't use it as a routine commute, but in a pinch it might work out.