B-D+D-B
"B-D+D-B" means there are two sets of articulated axles under the unit. Within each of these sets, there is a truck with two powered axles, and inboard of it are four powered axles. Two of these articulated sets are placed back to back and connected by a hinge. The W1 class of electric locomotives built by General Electric for the Great Northern Railway used this arrangement.
Read more about this topic: AAR Wheel Arrangement