The Centennial is also known as the DD-A-40X, which means that it rides on "D Trucks" (4 axles, 8 wheels per truck) which gives it 16 wheels.
"A" means that it has a cab, unlike the earlier DD 35 which was a cabless booster and, as Catt said, it was two GP40's mounted on a very long frame and was rated at 6600 horsepower.
Cheers.
Terry