what is the difference between a GP40 and an SD40? - Trains Magazine

 Just to confuse things EMD is currently offering a variant of it's SD70ACE unit with only 4 of the 6 axles powered, 2 are idler axles making a B1-1B wheel arrangement but the model will retain the "SD" designation..

Are these actually a B1-1B configuration, or an A-1-A, A-1A arrangement, like on the old E and PA units?

Doug