B-B

"B-B" means a locomotive (pure electric, diesel-mechanical, or diesel-electric) has two identical trucks. Each truck has two powered axles, a currently popular configuration used in high-speed, low-weight applications such as intermodal trains and high-speed rail, as well as in switching. An American Colloquialism of "B-B" is "Four axle". (From https://en.wikipedia.org/wiki/AAR_wheel_arrangement#B-B)