I have found for the last 25 years that spray paint from cans - hardware store or hobby shop Testors - are capable of paint jobs as nice as any from an airbrush, whether for passenger car sides or anything else, to be lettered or not. Others seem to agree about my work in general.
I've used both and decorated a lot of pieces, and haven't found the need for an airbrush very often. The last time I used one was for weathering, and I'm turning to washes, brushing and dry pigments even for that.
If I painted a lot of those gaudy 1st-Gen diesels an airbrush would probably get more use - odd color matching/mixing, and the like. But spray cans, especially with flat paint, equal airbrushing, at least in basic colors.
Atlas RS-1: spray-can red and aluminum; dry-brushed trucks; decals; spray-can Dullcote: