Why is there a gap in the AC4400CW's?

They are numbered from 1-173, and then 201-599. 5101-5122 also. Why no 174-200?

The intent was to seperate the rigid truck units from the steerable truck ones. (although a small number of the lowered number units have been retrofitted with steerable trucks) Also, the 600 series was filled up with the CW60AC's - which is why there is a group up in the 5100's.

That would make sense. Thanks for the clarification.