A couple of points: First diesels added dynamic brakes to the picture, something steam couldn't offer (other than the water brake that was less than effective). Secondly, I was at a convention last week and was talking to a motive power guy from a class one, he mentioned that with all the enviromental requirements hitting rebuilt locomotives too, and the required cab upgrades so that somehow people won't get hurt in a head on (like that's really possible), its rapidly becoming too costly to recycle old diesels by upgrading and rebuilding them, he says that we have reached the point where it's more cost effective to scrap units rather then rebuilding them, branch line power, i.e. 4 axle units, are just going to be out of luck, as with all the eco and crash requirements, a full fledged 4 axle (non-gen set or hybrid things) would be too heavey to run.