Modeling slugs it quite simple, but I'll have you know, they DO have doors, as they still do have electical components inside! I recently modeled BNSF 3967, a Santa Fe built 2 axle slug, built from a Fmr UP SD24B (Heck I even used an Atlas UP SD24B!!). I sawed the nose to the height needed, then chopped the sides down to the walkways. I then built up the sides with styrene, and added a roof. After completeing it, I realized they have 4 doors per side, two in the middle and one on either end. Not that you can't do it with out the doors, heck, I did! I added diamond tread walkways, and trimmed a-line steps (Sd40/45 cut to fit). It looks awesome, but the older atlas drive is too tall, so I'm going to add NWSL PDT's under her.