Yes, the rail from Fort Worth arrives at high B Gates. I have not walked from B to D, but since I was able to walk from A to C, I assume this is quite doable, and from memory reading here on Flyertalk there are connections from b2d. You cannot walk between terminals outside this I know. And whatever you do stay away from the shuttles outside they are horrible and slow. You can check in at any of the terminals and just take Skylink to where you need to be.