Another interesting tidbit about Rail King is that some of the rolling stock items are scale sized models of smaller prototypes, so mixing them with other scale items is not an issue.
As Pro Hobby posted, some of the old Premier items are now sold in the Rail King line. Several cabooses come to mind. Several Proto-2 and Proto-3 equpped Rail King diesel locomotives (GP7/GP9, SD9. RS27, C30-7, SD45, FP45, GP20, GP30, SD60) are former Premier as they originally could all handle O-31 curves.