I've done two complete locomotives now by brush, with the third planned for this summer. Brushes can get into spots that spraying can't. Also, because of overspray, things you don't want black (or green) you need to tape and mask, making for extra work. The alkyd enamel goes on well, doesn't attract much dust, dries quickly, and is pretty durable. The paint fumes are obnoxious, though, and I had a big fan going all the time. It just takes a long time to hand-brush; #12 took an entire day to prep, and then 6-7 Saturdays to paint. I was able to save some time by rolling much of the cab and tank sides.
Mike