I did frequent decal work on prototype 40ft. transit buses. The decal procedures for prototype railroad, truck, and aircraft is similar. We used silkscreen and vinyl materials. After a few months of practice, it became quite easy for me (though I always had to pay very close attention). My tools for most decal jobs were: Tape measure, 1-inch 3M masking tape, sharp cutting blades, a level, plastic rubberized squeeges, spray bottle with soapy water, and a T-square. Now for the vinyl graphics that cover vehicles, yes that is much more involved.
Anyway, the point of this is that, IMHO, decaling model railroad locomotives and cars is much more challenging. Getting those bar codes, ICC Trustee plates, Fire Extinguisher, number boards, and countless other tiny decals in place can be quite a nerve tasking job! [8)]
The original articulated double stacks that ACF built for Southern Pacific had the units numbered. I wasn’t sure if the lettering of the units had become a standard.