D on coach cars

what does the Yellow D next to the numbers on the side of a coach car stand for? i saw this yesterday at South Station on a double decker and this double decker was right behind the loco, does the Yellow D stand for that car can be near the engine?

This means an automatic door, especially for the old Colony Lines, which has been previously answered.

oh ok thank you

Anytime, Matt.

Now, his question has been answered and it's now closed.