NS and Conrail before the Conrail split were in pretty good shape. I think all the NW and SOU painted engines were off the roster. Conrail may not qualify as they had not painted all units into the Conrail Quality scheme.

When will it happen again? I don't know, given the way companies change their images. BNSF has changed images at least 4 times since it was created. UP has changed several times post SP from We Can Handle It to the big flag to Building America, wings, and lightning stripes, although the basic scheme and colors have remained the same. Through all the changes however, they have maintained a relatively uniform look. I actually like a little variety within an otherwise uniform overall scheme. CP went to the Golden Beaver, now has retired the beaver. CSX has gone to dark future, and NS to the Horsehead. KCS went from Gray to Southern Belle. CN has been fairly constant, but still has acquiree schemes running around. There aren't even many regionals with a uniform roster! It may be a long time before we see stability again, especially depending on how particular you are in the definition of it.