B-B+B-B
"B-B+B-B" means that the locomotive has four trucks. Each truck contains two powered axles. The middle pair of trucks are connected by a span bolster. In most cases the locomotive is articulated over the span bolster. The Union Pacific's M-10002 is an example of this type. This arrangement also includes locomotives made of two permanently coupled B-B units, such as some EMD FT units which had a solid drawbar connecting two units instead of the typical couplers.
Read more about this topic: AAR Wheel Arrangement