I helped built that box car while I was still in the car shop. It's built from flat car 6634.
This new car was built for a few reasons. The 3000s box cars are showing their age( even with a steel reinforced frame) and let's face it are pretty small. At some point the 3000s will need a major rebuild. So the idea of a new bigger, steel car came about.
All steel frame means unrestricted, as in no risk of damaging the car if placed at the head end.
They also wanted more room for fire fighting equipment, rafter equipment, backpackers stuff, Christmas trees, ect. Funny thing is everyone doesn't seem to understand that to get bigger on the inside you have to make it bigger on the out side. So after we built it I kept hearing complaints about it being to big.
Anyway. The roof walk is just for looks and the "ladders" are on the B end only. The ladders are for the brakemen climb to a small platform to reach the Ajax bake wheel.