I'm currently working on super-detailing an Atlas ALCO S-2 and an Atlas C424 Phase III.
What is super-detailing? Well, allow me to explain.
Super-detailing essentially means to add details to a model above and beyond what the model comes with. For example, most model engines have molded on details like grab irons (the "ladder" that goes to the roof of the engine), lift rings, bells and other things. When you super-detail a model, a lot of times you have to remove the original, molded on details. This requires filing, sanding, grinding, filling and drilling. The object is to replicate a prototype piece of equipment by removing the details that aren't needed and adding details that are.
Some people may ask, "Why do you go through all of that trouble? Why don't you just leave it the way it came?" For me, it has to do with recreating the look of an actual engine that was owned by the railroad that I am modelling, the Green Bay & Western. While Atlas makes really good looking and great running engines, they don't look exactly like the one the GBW owned. To some extent, railroads modified their engines to suit their needs. Likewise, model railroaders do the same thing via super-detailing.
My S-2 project required the removal of the grills on the side of the engine, all of the grab irons and the smoke stack. I've added, not only new grab irons and a smoke stack, but also some custom made vent covers, hand rails on the roof of the hood and a new bell. I'll also be adding brake chains under the cab and a rotary light to the top of the cab after painting.
Another on going project is the addition of DCC decoders to all of my locomotives. While this has been made easier by the ever increasing availability of drop-in decoders, I still have to install them. Also, the decoders that have sound capabilities require installing a speaker in addition to the decoder. Anybody who has experience with models of ALCO engines knows the problem with installing sound equipment in the narrow hoods of these units. The space is extremely limited so it's necessary to be somewhat inventive in how you install your speakers.